2011 |
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Title: Gaussian representation of extended quantum states
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| Author(s): Carvalho, ARR (Carvalho, A. R. R.); Kenfack, A (Kenfack, A.); Toscano, F (Toscano, F.); Rost, JM (Rost, J. M.); de Almeida, AMO (Ozorio de Almeida, A. M.) |
| Source: PHYSICS LETTERS A 37, pages: 19-23 DOI: 10.1016/j.physleta.2011.10.029 Published: NOV 28 2011 |
| Abstract: Excited energy eigenstates and their superpositions typically exhibit a fine oscillatory structure near caustics. Semiclassical theory accesses these, but depends on detailed geometrical knowledge of the caustics. Here we show that a finite placement of coherent states on the classical region efficiently fits such extended states, reproducing structures that are much finer than the Gaussian width of the basis states. An extended state, evolved such that it becomes fully distinguishable from the original state, can also be faithfully reproduced by this finite basis. The ideal fitting follows from the projection of the extended state on the finite Hilbert space spanned by the Gaussians, rather than any discretization of the continuous (overcomplete) coherent state representation. (C) 2011 Elsevier B.V. All rights reserved. |
| ISSN: 0375-9601 |
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Title: A Homonuclear Molecule with a Permanent Electric Dipole Moment
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| Author(s): Li, W (Li, W.); Pohl, T (Pohl, T.); Rost, JM (Rost, J. M.); Rittenhouse, ST (Rittenhouse, Seth T.); Sadeghpour, HR (Sadeghpour, H. R.); Nipper, J (Nipper, J.); Butscher, B (Butscher, B.); Balewski, JB (Balewski, J. B.); Bendkowsky, V (Bendkowsky, V.); Low, R (Loew, R.); Pfau, T (Pfau, T.) |
| Source: SCIENCE 334 Issue: 6059 Pages: 1110-1114 DOI: 10.1126/science.1211255 Published: NOV 25 2011 |
| Abstract: Permanent electric dipole moments in molecules require a breaking of parity symmetry. Conventionally, this symmetry breaking relies on the presence of heteronuclear constituents. We report the observation of a permanent electric dipole moment in a homonuclear molecule in which the binding is based on asymmetric electronic excitation between the atoms. These exotic molecules consist of a ground-state rubidium (Rb) atom bound inside a second Rb atom electronically excited to a high-lying Rydberg state. Detailed calculations predict appreciable dipole moments on the order of 1 Debye, in excellent agreement with the observations. |
| ISSN: 0036-8075 |
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Title: Coulomb explosion of uniformly charged spheroids
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| Author(s): Grech, M (Grech, M.); Nuter, R (Nuter, R.); Mikaberidze, A (Mikaberidze, A.); Di Cintio, P (Di Cintio, P.); Gremillet, L (Gremillet, L.); Lefebvre, E (Lefebvre, E.); Saalmann, U (Saalmann, U.); Rost, JM (Rost, J. M.); Skupin, S (Skupin, S.) |
| Source: PHYSICAL REVIEW E 84 Issue: 5 Article Number: 056404 DOI: 10.1103/PhysRevE.84.056404 Part: Part 2 Published: NOV 4 2011 |
| Abstract: A simple, semianalytical model is proposed for nonrelativistic Coulomb explosion of a uniformly charged spheroid. This model allows us to derive the time-dependent particle energy distributions. Simple expressions are also given for the characteristic explosion time and maximum particle energies in the limits of extreme prolate and oblate spheroids as well as for the sphere. Results of particle simulations are found to be in remarkably good agreement with the model. |
| ISSN: 1539-3755 |
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Title: Dopant-Induced Ignition of Helium Nanodroplets in Intense Few-Cycle Laser Pulses
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| Author(s): Krishnan, SR (Krishnan, S. R.); Fechner, L (Fechner, L.); Kremer, M (Kremer, M.); Sharma, V (Sharma, V.); Fischer, B (Fischer, B.); Camus, N (Camus, N.); Jha, J (Jha, J.); Krishnamurthy, M (Krishnamurthy, M.); Pfeifer, T (Pfeifer, T.); Moshammer, R (Moshammer, R.); Ullrich, J (Ullrich, J.); Stienkemeier, F (Stienkemeier, F.); Mudrich, M (Mudrich, M.); Mikaberidze, A (Mikaberidze, A.); Saalmann, U (Saalmann, U.); Rost, JM (Rost, J. -M.) |
| Source: PHYSICAL REVIEW LETTERS 107 Issue: 17 Article Number: 173402 DOI: 10.1103/PhysRevLett.107.173402 Published: OCT 19 2011 |
| Abstract: We demonstrate ultrafast resonant energy absorption of rare-gas doped He nanodroplets from intense few-cycle ( similar to 10 fs) laser pulses. We find that less than 10 dopant atoms "ignite'' the droplet to generate a nonspherical electronic nanoplasma resulting ultimately in complete ionization and disintegration of all atoms, although the pristine He droplet is transparent for the laser intensities applied. Our calculations at those intensities reveal that the minimal pulse length required for ignition is about 9 fs. |
| ISSN: 0031-9007 |
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Title: Lifetimes of ultralong-range Rydberg molecules in vibrational ground and excited states
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| Author(s): Butscher, B (Butscher, Bjoern); Bendkowsky, V (Bendkowsky, Vera); Nipper, J (Nipper, Johannes); Balewski, JB (Balewski, Jonathan B.); Kukota, L (Kukota, Ludmila); Low, R (Loew, Robert); Pfau, T (Pfau, Tilman); Li, WB (Li, Weibin); Pohl, T (Pohl, Thomas); Rost, JM (Rost, Jan Michael) |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 44 Issue: 18 Special Issue: SI Article Number: 184004 DOI: 10.1088/0953-4075/44/18/184004 Published: SEP 28 2011 |
| Abstract: Since their first experimental observation, ultralong-range Rydberg molecules consisting of a highly excited Rydberg atom and a ground state atom [1, 2] have attracted the interest in the field of ultracold chemistry [3, 4]. Especially the intriguing properties such as size, polarizability and type of binding they inherit from the Rydberg atom are of interest. An open question in the field is the reduced lifetime of the molecules compared to the corresponding atomic Rydberg states [2]. In this paper we present an experimental study on the lifetimes of the (3)Sigma (5s - 35s) molecule in its vibrational ground state and in an excited state. We show that the lifetimes depend on the density of ground state atoms and that this can be described in the frame of a classical scattering between the molecules and ground state atoms. We also find that the excited molecular state has an even more reduced lifetime compared to the ground state which can be attributed to an inward penetration of the bound atomic pair due to imperfect quantum reflection that takes place in the special shape of the molecular potential [5]. |
| ISSN: 0953-4075 |
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Title: Adiabatic entanglement transport in Rydberg aggregates
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| Author(s): Mobius, S (Moebius, S.); Wuster, S (Wuester, S.); Ates, C (Ates, C.); Eisfeld, A (Eisfeld, A.); Rost, JM (Rost, J. M.) |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 44 Issue: 18 Special Issue: SI Article Number: 184011 DOI: 10.1088/0953-4075/44/18/184011 Published: SEP 28 2011 |
| Abstract: We consider the interplay between excitonic and atomic motion in a regular, flexible chain of Rydberg atoms, extending our recent results on entanglement transport in Rydberg chains (Wuster et al 2010 Phys. Rev. Lett. 105 053004). In such a Rydberg chain, similar to molecular aggregates, an electronic excitation is delocalized due to long-range dipole-dipole interactions among the atoms. The transport of an exciton that is initially trapped by a chain dislocation is strongly coupled to nuclear dynamics, forming a localized pulse of combined excitation and displacement. This pulse transfers entanglement between dislocated atoms adiabatically along the chain. Details about the interaction and the preparation of the initial state are discussed. We also present evidence that the quantum dynamics of this complex many-body problem can be accurately described by selected quantum-classical methods, which greatly simplify investigations of excitation transport in flexible chains. |
| ISSN: 0953-4075 |
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Title: Monitoring Atomic Cluster Expansion by High-Harmonic Generation
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| Author(s): Strelkov, V (Strelkov, Vasily); Saalmann, U (Saalmann, Ulf); Becker, A (Becker, Andreas); Rost, JM (Rost, Jan M.) |
| Source: PHYSICAL REVIEW LETTERS 107 Issue: 11 Article Number: 113901 DOI: 10.1103/PhysRevLett.107.113901 Published: SEP 9 2011 |
| Abstract: High-harmonic generation is shown to be capable of providing time-resolved information about the particle density of a complex system. As an example, we study numerically high-harmonic generation from expanding xenon clusters in a pump-probe laser scheme, where the pump laser pulse induces the cluster explosion and the probe pulse generates harmonics in the expanding cluster. We show that the high-harmonic spectra characterize the properties of the expanding cluster. Hence, measuring the dependence of the harmonic signal on the pump-probe delay suggests itself as an experimental tool to monitor many-particle dynamics with unique temporal resolution; based on optical measurements, this technique is naturally free from any spatial charge effects. |
| ISSN: 0031-9007 |
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Title: Excitation transport through Rydberg dressing
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| Author(s): Wuster, S (Wuester, S.); Ates, C (Ates, C.); Eisfeld, A (Eisfeld, A.); Rost, JM (Rost, J. M.) |
| Source: NEW JOURNAL OF PHYSICS 13 Article Number: 073044 DOI: 10.1088/1367-2630/13/7/073044 Published: JUL 29 2011 |
| Abstract: We show how to create long-range interactions between alkali atoms in different hyperfine ground states, with the goal of coherent quantum transport. The scheme uses off-resonant dressing with atomic Rydberg states. We demonstrate coherent migration of electronic excitation through dressed dipole-dipole interaction by full solutions of models with four essential states per atom and give the structure of the spectrum of dressed states for a dimer. In addition, we present an effective (perturbative) Hamiltonian for the ground-state manifold and show that it correctly describes the full multi-state dynamics. We discuss excitation transport in detail for a chain of five atoms. In the presented scheme, the actual population in the Rydberg state is kept small. Dressing offers many advantages over the direct use of Rydberg levels: it reduces ionization probabilities and provides an additional tuning parameter for lifetimes and interaction strengths. |
| ISSN: 1367-2630 |
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Title: Mixed quantum-classical approach to multiphoton dissociation of the hydrogen molecular ion
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| Author(s): Fischer, M (Fischer, M.); Grossmann, F (Grossmann, F.); Schmidt, R (Schmidt, R.); Handt, J (Handt, J.); Krause, SM (Krause, S. M.); Rost, JM (Rost, J-M) |
| Source: NEW JOURNAL OF PHYSICS 13 Article Number: 053019 DOI: 10.1088/1367-2630/13/5/053019 Published: MAY 13 2011 |
| Abstract: We present a mixed quantum-classical propagation method for the time-dependent dynamics of nuclear as well as electronic degrees of freedom for para-H(2)(+) exposed to short intense laser pulses of 800 nm wavelength. Depending on the initial vibrational state, the angular distributions of photofragments show characteristic shapes in very good agreement with our full-dimensional nuclear dynamics quantum calculations. The results can be understood in terms of two-dimensional adiabatic Floquet surfaces, which depend on the internuclear separation and rotation angle, demonstrating that adiabatic light-dressed surfaces are also a useful concept for full-dimensional nuclear dynamics. Using kinetic energy release spectra, we are also able to extract the contributions of different photon channels in the quantum-classical case. |
| ISSN: 1367-2630 |
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Title: Conical Intersections in an Ultracold Gas
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| Author(s): Wuster, S (Wuester, S.); Eisfeld, A (Eisfeld, A.); Rost, JM (Rost, J. M.) |
| Source: PHYSICAL REVIEW LETTERS 106 Issue: 15 Article Number: 153002 DOI: 10.1103/PhysRevLett.106.153002 Published: APR 12 2011 |
| Abstract: We find that energy surfaces of more than two atoms or molecules interacting via transition dipole-dipole potentials generically possess conical intersections (CIs). Typically only few atoms participate strongly in such an intersection. For the fundamental case, a circular trimer, we show how the CI affects adiabatic excitation transport via electronic decoherence or geometric phase interference. These phenomena may be experimentally accessible if the trimer is realized by light alkali atoms in a ring trap, whose interactions are induced by off-resonant dressing with Rydberg states. Such a setup promises a direct probe of the full many-body density dynamics near a CI. |
| ISSN: 0031-9007 |
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Title: Dynamics of photo-activated Coulomb complexes
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| Author(s): Gnodtke, C (Gnodtke, Christian); Saalmann, U (Saalmann, Ulf); Rost, JM (Rost, Jan-Michael) |
| Source: NEW JOURNAL OF PHYSICS 13 Article Number: 013028 DOI: 10.1088/1367-2630/13/1/013028 Published: JAN 2011 |
| Abstract: Intense light with frequencies above typical atomic or molecular ionization potentials as provided by free-electron lasers couples many photons into extended targets such as clusters and biomolecules. This implies, in contrast to traditional multi-photon ionization, multiple single-photon absorption. Thereby many electrons are removed from their bound states and either released or trapped if the target charge has become sufficiently large. We develop a simple model for this photo-activation to study electron migration and interaction. It satisfies scaling relations that help to relate quite different scenarios. Understanding this multi-electron dynamics on very short time scales is vital for assessing the radiation damage inflicted and for paving the way to coherent diffraction imaging of single molecules. |
| ISSN: 1367-2630 |
2010 |
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Title: Intense X-ray science: the first 5 years of FLASH
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| Author(s): Chapman, H (Chapman, H.); Ullrich, J (Ullrich, J.); Rost, JM (Rost, J. M.) |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 43 Issue: 19 Special Issue: SI Article Number: 190201 DOI: 10.1088/0953-4075/43/19/190201 Published: OCT 14 2010 |
| ISSN: 0953-4075 |
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Title: Rydberg Trimers and Excited Dimers Bound by Internal Quantum Reflection
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| Author(s): Bendkowsky, V (Bendkowsky, V.); Butscher, B (Butscher, B.); Nipper, J (Nipper, J.); Balewski, JB (Balewski, J. B.); Shaffer, JP (Shaffer, J. P.); Low, R (Loew, R.); Pfau, T (Pfau, T.); Li, W (Li, W.); Stanojevic, J (Stanojevic, J.); Pohl, T (Pohl, T.); Rost, JM (Rost, J. M.) |
| Source: PHYSICAL REVIEW LETTERS 105 Issue: 16 Article Number: 163201 DOI: 10.1103/PhysRevLett.105.163201 Published: OCT 12 2010 |
| Abstract: In a combined experimental and theoretical effort we report on two novel types of ultracold long-range Rydberg molecules. First, we demonstrate the creation of triatomic molecules of one Rydberg atom and two ground-state atoms in a single-step photoassociation. Second, we assign a series of excited dimer states that are bound by a so far unexplored mechanism based on internal quantum reflection at a steep potential drop. The properties of the Rydberg molecules identified in this work qualify them as prototypes for a new type of chemistry at ultracold temperatures. |
| ISSN: 0031-9007 |
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Title: Atomic and molecular systems driven by intense random light
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| Author(s): Singh, KP (Singh, Kamal P.); Rost, JM (Rost, Jan M.) |
| Source: CHEMICAL PHYSICS 375 Issue: 2-3 Pages: 144-149 DOI: 10.1016/j.chemphys.2010.08.003 Published: OCT 5 2010 |
| Abstract: We investigate dynamics of atomic and molecular systems exposed to intense, shaped random fields and a weak femtosecond laser pulse theoretically. As a prototype example, the photoionization of a hydrogen atom is considered in detail. The net photoionization undergoes an optimal enhancement when a broad-band random field is added to the weak laser pulse. The enhanced ionization is analyzed using time-resolved wavepacket evolution and the population dynamics of the atomic levels. We elucidate the enhancement produced by spectrally shaped random fields of two different classes, one with a tunable bandwidth and another with a narrow bandwidth centered at the first atomic transition. Motivated by the large bandwidth provided in the high harmonic generation, we also demonstrate the enhancement effect exploiting random fields synthesized from discrete, phase randomized, odd-order and all-order high harmonics of the driving pulse. These findings are generic and can have applications to other atomic and simple molecular systems. (C) 2010 Elsevier B. V. All rights reserved. |
| ISSN: 0301-0104 |
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Title: Fast electrons from multi-electron dynamics in xenon clusters induced by inner-shell ionization
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| Author(s): Bostedt, C (Bostedt, Christoph); Thomas, H (Thomas, Heiko); Hoener, M (Hoener, Matthias); Moller, T (Moeller, Thomas); Saalmann, U (Saalmann, Ulf); Georgescu, I (Georgescu, Ionut); Gnodtke, C (Gnodtke, Christian); Rost, JM (Rost, Jan-Michael) |
| Source: NEW JOURNAL OF PHYSICS 12 Article Number: 083004 DOI: 10.1088/1367-2630/12/8/083004 Published: AUG 3 2010 |
| Abstract: Fast electrons emitted from xenon clusters in strong femtosecond 90 eV pulses have been measured at the Free-electron Laser in Hamburg (FLASH). Energy absorption occurs mainly through atomic inner-shell photo-ionization. Photo-electrons are trapped in the strong Coulomb potential of the cluster ions and form a non-equilibrium plasma with supra-atomic density. Its equilibration through multiple energy-exchanging collisions within the entire cluster volume produces electrons with energies well beyond the dominant emission line of atomic xenon. Here, in contrast to traditional low-frequency laser plasma heating, the plasma gains energy from electrons delivered through massive single-photon excitation from bound states. Electron emission induced by thermalization of a non-equilibrium plasma is expected to be a general phenomenon occurring for strong atomic x-ray absorption in extended systems. |
| ISSN: 1367-2630 |
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Title: Newton's Cradle and Entanglement Transport in a Flexible Rydberg Chain
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| Author(s): Wuster, S (Wuester, S.); Ates, C (Ates, C.); Eisfeld, A (Eisfeld, A.); Rost, JM (Rost, J. M.) |
| Source: PHYSICAL REVIEW LETTERS 105 Issue: 5 Article Number: 053004 DOI: 10.1103/PhysRevLett.105.053004 Published: JUL 28 2010 |
| Abstract: In a regular, flexible chain of Rydberg atoms, a single electronic excitation localizes on two atoms that are in closer mutual proximity than all others. We show how the interplay between excitonic and atomic motion causes electronic excitation and diatomic proximity to propagate through the Rydberg chain as a combined pulse. In this manner entanglement is transferred adiabatically along the chain, reminiscent of momentum transfer in Newton's cradle. |
| ISSN: 0031-9007 |
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Title: Suppression of Exponential Electronic Decay in a Charged Environment
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| Author(s): Averbukh, V (Averbukh, Vitali); Saalmann, U (Saalmann, Ulf); Rost, JM (Rost, Jan Michael) |
| Source: PHYSICAL REVIEW LETTERS 104 Issue: 23 Article Number: 233002 DOI: 10.1103/PhysRevLett.104.233002 Published: JUN 7 2010 |
| Abstract: Inner-shell ionization of atoms and molecules leads to the creation of highly excited ionic states that often decay by electron emission. The dynamics of the decay is usually assumed to be exponential and the process is characterized by a decay rate. Here we show that in a multiply ionized cluster created by interaction with a high-intensity free-electron laser (FEL) radiation, trapping of the emitted electron by the neighboring ions changes the character of the decay dynamics qualitatively to the extent that it can become oscillatory instead of exponential. Implications of the predicted effect on Coster-Kronig and interatomic Coulombic decay processes induced by FELs are investigated. |
| ISSN: 0031-9007 |
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Title: Anomalous quantum reflection as a quasidynamical damping effect
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| Author(s): Jurisch, A (Jurisch, Alexander); Rost, JM (Rost, Jan-Michael) |
| Source: PHYSICAL REVIEW A 81 Issue: 4 Article Number: 043610 DOI: 10.1103/PhysRevA.81.043610 Published: APR 2010 |
| Abstract: We develop a quasianalytical theory for the quantum reflection amplitude of Bose-Einstein condensates. We derive and calculate the decay width of a Bose-Einstein condensate (BEC). A general relation between the time-dependent decay law of the system and its quantum reflection amplitude allows us to explain the quantum reflection anomaly of Bose-Einstein condensates present in BEC-surface systems as a direct consequence of the repulsive particle interaction. |
| ISSN: 1050-2947 |
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Title: Reliability of soft-core approximations in theoretical studies of molecules in intense laser fields
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| Author(s): Kastner, A (Kaestner, Alexander); Grossmann, F (Grossmann, Frank); Schmidt, R (Schmidt, Ruediger); Rost, JM (Rost, Jan-Michael) |
| Source: PHYSICAL REVIEW A 81 Issue: 2 Article Number: 023414 DOI: 10.1103/PhysRevA.81.023414 Published: FEB 2010 |
| Abstract: The popular approach of softening the Coulomb singularity in models with reduced dimensionality leads to uncontrollable discrepancies with numerical results obtained using exact potentials. This is demonstrated for ionization and dissociation probabilities as well as for high-order harmonic generation spectra even of the simple hydrogen molecular ion in aligned geometry. Soft-core models perform better in full dimensions, thereby losing one of their advantages, namely the gain in computation time that is achieved by going to reduced dimensionality. However, the much simpler numerical implementation of soft-core potentials carries over to full dimensions. Moreover, a generalization of the soft-core potential given in previous work [M. Lein, P. P. Corso, J. P. Marangos, and P. L. Knight, Phys. Rev. A 67, 023819 (2003)] is proposed, leading to a tremendous improvement toward the exact results, which is very promising for future calculations without cylindrical symmetry. |
| ISSN: 1050-2947 /td> |
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Title: Correlations of Rydberg excitations in an ultracold gas after an echo sequence
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| Author(s): Wuster, S (Wuester, S.); Stanojevic, J (Stanojevic, J.); Ates, C (Ates, C.); Pohl, T (Pohl, T.); Deuar, P (Deuar, P.); Corney, JF (Corney, J. F.); Rost, JM (Rost, J. M.) |
| Source: PHYSICAL REVIEW A 81 Issue: 2 Article Number: 023406 DOI: 10.1103/PhysRevA.81.023406 Published: FEB 2010 |
| Abstract: We show that Rydberg states in an ultracold gas can be excited with strongly preferred nearest-neighbor distance if densities are well below saturation. The scheme makes use of an echo sequence in which the first half of a laser pulse excites Rydberg states while the second half returns atoms to the ground state, as in the experiment of Raitzsch et al. [Phys. Rev. Lett. 100, 013002 (2008)]. Near the end of the echo sequence, almost any remaining Rydberg atom is separated from its next-neighbor Rydberg atom by a distance slightly larger than the instantaneous blockade radius halfway through the pulse. These correlations lead to large deviations of the atom-counting statistics from a Poissonian distribution. Our results are based on the exact quantum evolution of samples with small numbers of atoms. Finally, we demonstrate the utility of the. expansion for the approximate description of correlation dynamics through an echo sequence. |
| ISSN: 1050-2947 |
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Title: ATOMIC VAPOUR MICROCELLS Tubes for quantum electronics
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| Author(s): Rost, JM (Rost, Jan-Michael) |
| Source: NATURE PHOTONICS 4 Issue: 2 Pages: 74-75 DOI: 10.1038/nphoton.2009.279 Published: FEB 2010 |
| ISSN: 1749-4885 |
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Title: Photoionization of Xe inside C(60): Atom-fullerene hybridization, giant cross-section enhancement, and correlation confinement resonances
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| Author(s): Madjet, ME (Madjet, Mohamed E.); Renger, T (Renger, Thomas); Hopper, DE (Hopper, Dale E.); McCune, MA (McCune, Matthew A.); Chakraborty, HS (Chakraborty, Himadri S.); Rost, JM (Rost, Jan-M.); Manson, ST (Manson, Steven T.) |
| Source: PHYSICAL REVIEW A 81 Issue: 1 Article Number: 013202 DOI: 10.1103/PhysRevA.81.013202 Published: JAN 2010 |
| Abstract: A theoretical study of the subshell photoionization of the Xe atom endohedrally confined in C(60) is presented. Powerful hybridization of the Xe 5s state with the bottom edge of C(60) pi band is found that induces strong structures in the 5s ionization, causing the cross section to differ significantly from earlier results that omit this hybridization. The hybridization also affects the angular distribution asymmetry parameter of Xe 5p ionization near the Cooperminimum. The 5p cross section, on the other hand, is greatly enhanced by borrowing considerable oscillator strength from the C(60) giant plasmon resonance via the atom-fullerene dynamical interchannel coupling. Beyond the C(60) plasmon energy range the atomic subshell cross sections display confinement-induced oscillations in which, over the large 4d shape resonance region, the dominant 4d oscillations induce their "clones" in all degenerate weaker channels known as correlation confinement resonances. |
| ISSN: 1050-2947 |
2009 |
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Title: Steering a molecule into dissociation via vibrational excitation
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| Author(s): Kastner, A (Kaestner, Alexander); Grossmann, F (Grossmann, Frank); Krause, S (Krause, Sebastian); Schmidt, R (Schmidt, Ruediger); Kenfack, A (Kenfack, Anatole); Rost, JM (Rost, Jan-Michael) |
| Source: NEW JOURNAL OF PHYSICS 11 Article Number: 083014 DOI: 10.1088/1367-2630/11/8/083014 Published: AUG 13 2009 |
| Abstract: For a laser driven molecule, we show that the ionization and the dissociation channels can be separated by preparing the molecule in a specific vibrational state. Specifically, we investigate the dynamics of the hydrogen molecular ion under a femtosecond infrared laser field aligned with the molecular axis. We find dissociation probabilities of more than 60%, considerably higher than reported so far. We demonstrate that a full dimensional description of the electron dynamics is necessary to obtain accurate results for the combined ionization/dissociation dynamics. |
| ISSN: 1367-2630 |
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Title: Photoionization of hybrid states in endohedral fullerenes
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| Author(s): Chakraborty, HS (Chakraborty, Himadri S.); Madjet, ME (Madjet, Mohamed E.); Renger, T (Renger, Thomas); Rost, JM (Rost, Jan-M); Manson, ST (Manson, Steven T.) |
| Source: PHYSICAL REVIEW A 79 Issue: 6 Article Number: 061201 DOI: 10.1103/PhysRevA.79.061201 Published: JUN 2009 |
| Abstract: Considering a Xe atom endohedrally confined in C(60), we predict the formation of another type of atom-fullerene hybrid states. These dimer-type states arise from the near degeneracy of inner levels in contrast to the known overlap-induced hybrid states around the Fermi level of smaller compounds. The photoionization cross sections of these hybrid states exhibit rich structures and are radically different from the cross sections of free atomic or fullerene states. |
| ISSN: 1050-2947 |
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Title: Strong field dynamics with ultrashort electron wave packet replicas
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| Author(s): Riviere, P (Riviere, Paula); Uhden, O (Uhden, Olaf); Saalmann, U (Saalmann, Ulf); Rost, JM (Rost, Jan M.) |
| Source: NEW JOURNAL OF PHYSICS 11 Article Number: 053011 DOI: 10.1088/1367-2630/11/5/053011 Published: MAY 22 2009 |
| Abstract: We investigate theoretically electron dynamics under a vacuum ultraviolet (VUV) attosecond pulse train, which has a controlled phase delay with respect to an additional strong infrared laser field. Using the strong field approximation and the fact that the attosecond pulse is short compared to the excited electron dynamics, we arrive at a minimal analytical model for the kinetic energy distribution of the electron as well as the photon absorption probability as a function of the phase delay between the fields. We analyze the dynamics in terms of electron wave packet replicas created by the attosecond pulses. The absorption probability shows strong modulations as a function of the phase delay for VUV photons of energy comparable to the binding energy of the electron, while for higher photon energies the absorption probability does not depend on the delay, in line with the experimental observations for helium and argon, respectively. |
| ISSN: 1367-2630 |
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Title: Ultra-Long-Range Rydberg Trimers with a Repulsive Two-Body Interaction
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| Author(s): Liu, ICH (Liu, Ivan C. H.); Stanojevic, J (Stanojevic, Jovica); Rost, JM (Rost, Jan M.) |
| Source: PHYSICAL REVIEW LETTERS 102 Issue: 17 Article Number: 173001 DOI: 10.1103/PhysRevLett.102.173001 Published: MAY 1 2009 |
| Abstract: A Rydberg and a ground-state atom can form ultra-long-range diatomic molecules provided the interaction between the ground-state atom and the Rydberg electron is attractive [C. H. Greene , Phys. Rev. Lett. 85, 2458 (2000)]. A repulsive interaction does not support bound states. However, as we will show, adding a second ground-state atom, a long-range bound triatomic molecule becomes possible. |
| ISSN: 0031-9007 |
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Title: Ionization and charge migration through strong internal fields in clusters exposed to intense x-ray pulses
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| Author(s): Gnodtke, C (Gnodtke, Christian); Saalmann, U (Saalmann, Ulf); Rost, JM (Rost, Jan M.) |
| Source: PHYSICAL REVIEW A 79 Issue: 4 Article Number: 041201 DOI: 10.1103/PhysRevA.79.041201 Published: APR 2009 |
| Abstract: A general scenario for electronic charge migration in finite samples illuminated by an intense laser pulse is given. Microscopic calculations for neon clusters under strong short pulses as produced by x-ray free-electron laser sources confirm this scenario and point to the prominent role of field ionization by strong internal fields. The latter leads to the fast formation of a core-shell system with an almost static core of screened ions while the outer shell explodes. Substituting the shell ions with a different material such as helium as a sacrificial layer leads to a substantial improvement of the diffraction image for the embedded cluster thus reducing the consequences of radiation damage for coherent diffractive imaging. |
| ISSN: 1050-2947 |
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Title: Laser-Driven Nanoplasmas in Doped Helium Droplets: Local Ignition and Anisotropic Growth
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| Author(s): Mikaberidze, A (Mikaberidze, Alexey); Saalmann, U (Saalmann, Ulf); Rost, JM (Rost, Jan M.) |
| Source: PHYSICAL REVIEW LETTERS 102 Issue: 12 Article Number: 128102 DOI: 10.1103/PhysRevLett.102.128102 Published: MAR 27 2009 |
| Abstract: Doping a helium nanodroplet with only a tiny xenon cluster of a few atoms sparks complete ionization of the droplet at laser intensities below the ionization threshold of helium atoms. As a result, the intrinsically inert and transparent droplet turns into a fast and strong absorber of infrared light. Microscopic calculations reveal a two-step mechanism to be responsible for the dramatic change: Avalanchelike ionization of the helium atoms on a femtosecond time scale, driven by field ionization due to the quickly charged xenon core, is followed by resonant absorption enabled by an unusual cigar-shaped nanoplasma within the droplet. |
| ISSN: 0031-9007 |
| -- |
|
Title: Resonances: from few-body to many-body phenomena
|
| Author(s): Cederbaum, LS (Cederbaum, L. S.); Rost, JM (Rost, J-M); Sadeghpour, HR (Sadeghpour, H. R.) |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 42 Issue: 4 Article Number: 040201 DOI: 10.1088/0953-4075/42/4/040201 Published: FEB 28 2009 |
| ISSN: 0953-4075 |
2008 |
| Author(s): Chakraborty, HS (Chakraborty, Himadri S.); Madjet, ME (Madjet, Mohamed E.); Rost, JM (Rost, Jan-M.); Manson, ST (Manson, Steven T.) |
|
Title:
Dynamical effects of confinement on atomic valence photoionization in Mg@C-60 |
| Source: PHYSICAL REVIEW A, 78 (1): Art. No. 013201 JUL 2008 |
| Abstract: We predict a huge dynamical boost from collective electron motion on the valence 3s photoionization of Mg in the Mg@C-60 molecule. Calculations have been performed in a framework of time-dependent local density approximation that includes coupling between dipole-allowed final state channels. Our results qualitatively agree with the recent predictions of a semiclassical model, however, significantly differing in magnitude. In particular, a strong dependence on the nature of the caged atom is found which is at odds with the prediction of the semiclassical result. |
| ISSN: 1050-2947 |
| DOI: 10.1103/PhysRevA.78.013201 |
| -- |
| Author(s): Ates, C (Ates, C.); Eisfeld, A (Eisfeld, A.); Rost, JM (Rost, J. M.) |
|
Title:
Motion of Rydberg atoms induced by resonant dipole-dipole interactions |
| Source: NEW JOURNAL OF PHYSICS, 10: Art. No. 045030 APR 30 2008 |
| Abstract: We show that nuclear motion of Rydberg atoms can be induced by resonant dipole-dipole interactions that trigger the energy transfer between two energetically close Rydberg states. How and if the atoms move depends on their initial arrangement as well as on the initial electronic excitation. Using a mixed quantum/classical propagation scheme, we obtain the trajectories and kinetic energies of atoms, initially arranged in a regular chain and prepared in excitonic eigenstates. The influence of the off-diagonal disorder on the motion of the atoms is examined and it is shown that irregularity in the arrangement of the atoms can lead to an acceleration of the nuclear dynamics. |
| ISSN: 1367-2630 |
| DOI: 10.1088/1367-2630/10/4/045030 |
| -- |
| Author(s): Madjet, ME (Madjet, Mohamed E.); Chakraborty, HS (Chakraborty, Himadri S.); Rost, JM (Rost, Jan M.); Manson, ST (Manson, Steven T.) |
|
Title:
Photoionization of C-60: a model study |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 41 (10): Art. No. 105101 MAY 28 2008 |
| Abstract: Time-dependent density functional theory is used to calculate the total and subshell photoionization cross sections of C-60. The core of 60 C4+ ions is smeared into a classical jellium shell before treating the correlated motion of the 240 valence electrons quantum mechanically. The calculation reveals two collective plasmon resonances in the total cross section in agreement with the experiment. It is found that a phase-coherent superposition of amplitudes leading to enhancements in the ionization from various C-60 subshells in two distinct energy regions essentially builds the plasmons. While the result shows good qualitative agreement with the experiments, the limitation of the model to describe the data in quantitative detail is discussed. |
| ISSN: 0953-4075 |
| DOI: 10.1088/0953-4075/41/10/105101 |
| -- |
| Author(s): Mikaberidze, A (Mikaberidze, Alexey); Saalmann, U (Saalmann, Ulf); Rost, JM (Rost, Jan M.) |
|
Title:
Energy absorption of xenon clusters in helium nanodroplets under strong laser pulses |
| Source: PHYSICAL REVIEW A, 77 (4): Art. No. 041201 APR 2008 |
| Abstract: Energy absorption of xenon clusters embedded in helium nanodroplets from strong femtosecond laser pulses is studied theoretically. Compared to pure clusters we find earlier and more efficient energy absorption in agreement with experiments. This effect is due to resonant absorption of the helium nanoplasma whose formation is catalyzed by the xenon core. For very short double pulses with variable delay, two plasma resonances, due to the helium shell and the xenon core, are identified and the experimental conditions are given which should allow for a simultaneous observation of both of them. |
| ISSN: 1050-2947 |
| DOI: 10.1103/PhysRevA.77.041201 |
| -- |
| Author(s): Jurisch, A (Jurisch, Alexander); Rost, JM (Rost, Jan-Michael) |
|
Title:
Trapping cold atoms by quantum reflection |
| Source: PHYSICAL REVIEW A, 77 (4): Art. No. 043603 APR 2008 |
| Abstract: We examine the properties of a quantum reflection trap when particle interaction is included. We explore the influence of the particle interaction on the trapping for different regimes: repulsive particle interaction and attractive particle interactions in its stable and unstable limit. With variational techniques, we calculate the phase diagram of the quantum reflection trap and determine the stable and unstable regimes of the system. |
| ISSN: 1050-2947 |
| DOI: 10.1103/PhysRevA.77.043603 |
| -- |
| Author(s): Riviere, P (Riviere, Paula); Ruiz, C (Ruiz, Camilo); Rost, JM (Rost, Jan-Michael) |
|
Title:
Attosecond light-pulse-induced photoassociation (vol 77, artn 033421, 2008) |
| Source: PHYSICAL REVIEW A, 77 (4): Art. No. 049904 APR 2008 |
| ISSN: 1050-2947 |
| DOI: 10.1103/PhysRevA.77.049904 |
| -- |
| Author(s): Toscano, F (Toscano, Fabricio); Kenfack, A (Kenfack, Anatole); Carvalho, ARR (Carvalho, Andre R. R.); Rost, JM (Rost, Jan M.); de Almeida, AMO (Ozorio de Almeida, Alfredo M.) |
|
Title:
Husimi-Wigner representation of chaotic eigenstates |
| Source: PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, 464 (2094): 1503-1524 JUN 8 2008 |
|
Abstract:
Just as a coherent state may be considered as a quantum point, its restriction to a factor space of the full Hilbert space can be interpreted as a quantum plane. The overlap of such a factor coherent state with a full pure state is akin to a quantum section. It defines a reduced pure state in the cofactor Hilbert space. Physically, this factorization corresponds to the description of interacting components of a quantum system with many degrees of freedom and the sections could be generated by conceivable partial measurements. The collection of all the Wigner functions corresponding to a full set of parallel quantum sections defines the Husimi Wigner representation. It occupies an intermediate ground between the drastic suppression of non-classical features, characteristic of Husimi functions, and the daunting complexity of higher dimensional Wigner functions. After analysing these features for simpler states, we exploit this new representation as a probe of numerically computed eigenstates of a chaotic Hamiltonian. Though less regular, the individual two-dimensional Wigner functions resemble those of semiclassically quantized states. |
| ISSN: 1364-5021 |
| DOI: 10.1098/rspa.2007.0263 |
| -- |
| Author(s): Saalmann, U (Saalmann, Ulf); Rost, JM (Rost, Jan M.) |
|
Title:
Rescattering for extended atomic systems |
| Source: PHYSICAL REVIEW LETTERS, 100 (13): Art. No. 133006 APR 4 2008 |
| Abstract: Laser-driven rescattering of electrons is the basis of many strong-field phenomena in atoms and molecules. Here, we will show how this mechanism operates in extended atomic systems, giving rise to effective energy absorption. Rescattering from extended systems can also lead to energy loss, which in its extreme form results in nonlinear light-induced trapping. Intense-laser interaction with atomic clusters is discussed as an example. We explain fast electron emission, seen in experimental and numerically obtained spectra, by rescattering of electrons at the highly charged cluster. |
| ISSN: 0031-9007 |
| DOI: 10.1103/PhysRevLett.100.133006 |
| -- |
| Author(s): Riviere, P (Riviere, Paula); Ruiz, C (Ruiz, Camilo); Rost, JM (Rost, Jan-Michael) |
|
Title:
Attosecond light-pulse-induced photoassociation |
| Source: PHYSICAL REVIEW A, 77 (3): Art. No. 033421 MAR 2008 |
| Abstract: We explore stimulated photoassociation in the context of attosecond pump- probe schemes of atomic matter. An attosecond pulse - the probe - is used to induce photoassociation of an electronic wave packet which had been created before, typically with an attosecond pump pulse at an atomic center different from the one of photoassociation. We will show that the electron absorption is maximal for a certain delay between the pulses. Two ways of enhancing and controlling stimulated photoassociation are proposed, namely, using an additional infrared pulse to steer the electronic wave packet and using a train of attosecond pulses instead of a single pair. A direct application of ultrafast stimulated photoassociation is the measurement of atomic distances. |
| ISSN: 1050-2947 |
| DOI: 10.1103/PhysRevA.77.033421 |
| -- |
| Author(s): Singh, KP (Singh, Kamal P.); Kenfack, A (Kenfack, Anatole); Rost, JM (Rost, Jan M.) |
|
Title:
Femtosecond photodissociation of molecules facilitated by noise |
| Source: PHYSICAL REVIEW A, 77 (2): Art. No. 022707 FEB 2008 |
| Abstract: We investigate the dynamics of diatomic molecules subjected to both a femtosecond midinfrared laser pulse and Gaussian white noise. The stochastic Schrodinger equation with a Morse potential is used to describe the molecular vibrations under noise and the laser pulse. For weak laser intensity, well below the dissociation threshold, it is shown that one can find an optimum amount of noise that leads to a dramatic enhancement of the dissociation probability. The enhancement landscape, which is shown as a function of both the noise and the laser strength, exhibits a global maximum. A frequency-resolved gain profile is recorded with a pump-probe setup which is experimentally realizable. With this profile we identify the linear and nonlinear multiphoton processes created by the interplay between laser and noise and assess their relative contribution to the dissociation enhancement. |
| ISSN: 1050-2947 |
| DOI: 10.1103/PhysRevA.77.022707 |
| -- |
| Author(s): Saalmann, U (Saalmann, Ulf); Georgescu, I (Georgescu, Ionut); Rost, JM (Rost, Jan M.) |
|
Title:
Tracing non-equilibrium plasma dynamics on the attosecond timescale in small clusters |
| Source: NEW JOURNAL OF PHYSICS, 10: Art. No. 025014 FEB 29 2008 |
| Abstract: It is shown by microscopic calculations that the energy absorption of a rare-gas cluster from a vacuum-ultraviolet (VUV) pulse can be traced with time-delayed extreme-ultraviolet (XUV) attosecond probe pulses by measuring the kinetic energy of the electrons detached by the probe pulse. By means of this scheme we demonstrate that, for pump pulses as short as one femtosecond, the charging of the cluster proceeds during the formation of an electronic nano-plasma inside the cluster. Using moderate harmonics for the VUV and high harmonics for the XUV pulse from the same near-infrared laser source, this scheme with well defined time delays between pump and probe pulses should be experimentally realizable. Going to even shorter pulse durations we predict that pump and probe pulses of about 250 attoseconds can induce and monitor non-equilibrium dynamics of the nano-plasma. |
| ISSN: 1367-2630 |
| DOI: 10.1088/1367-2630/10/2/025014 |
| -- |
| Author(s): Emmanouilidou, A (Emmanouilidou, Agapi); Wang, PJ (Wang, Peijie); Rost, JM (Rost, Jan M.) |
|
Title:
Initial state dependence in multielectron threshold ionization of atoms |
| Source: PHYSICAL REVIEW LETTERS, 100 (6): Art. No. 063002 FEB 15 2008 |
| Abstract: It is shown that the geometry of multielectron threshold ionization in atoms depends on the initial configuration of bound electrons. The reason for this behavior is found in the stability properties of the classical fixed point of the equations of motion for multiple threshold fragmentation. Specifically for three-electron breakup, apart from the symmetric triangular configuration also a breakup of lower symmetry in the form of a inverted perpendicular shape can occur, as we demonstrate by calculating triple photoionization for the lithium ground and first excited states. We predict the electron breakup geometry for threshold fragmentation experiments. |
| ISSN: 0031-9007 |
| DOI: 10.1103/PhysRevLett.100.063002 |
| -- |
| Author(s): Kenfack, A (Kenfack, Anatole); Rost, JM (Rost, Jan M.); Grossmann, F (Grossmann, Frank) |
|
Title:
Dissociation and ionization of small molecules steered by external noise |
| Source: NEW JOURNAL OF PHYSICS, 10: Art. No. 013020 JAN 21 2008 |
| Abstract: We show that ionization and dissociation can be influenced separately in a molecule with appropriate external noise. Specifically, we investigate the hydrogen molecular ion under a stochastic force quantum mechanically beyond the Born-Oppenheimer approximation. We find that up to 30% of dissociation without ionization can be achieved by suitably tuning the forcing parameters. |
| ISSN: 1367-2630 |
2007 |
| Author(s): Singh, KP (Singh, Kamal P.); Rost, JM (Rost, Jan M.) |
|
Title:
Femtosecond photoionization of atoms under noise |
| Source: PHYSICAL REVIEW A, 76 (6): Art. No. 063403 DEC 2007 |
| Abstract: We investigate the effect of incoherent perturbations on atomic photoionization due to a femtosecond midinfrared laser pulse by solving the time-dependent stochastic Schrödinger equation. For a weak laser pulse which causes almost no ionization, an addition of a Gaussian white noise to the pulse leads to a significantly enhanced ionization probability. Tuning the noise level, a stochastic resonancelike curve is observed showing the existence of an optimum noise for a given laser pulse. Besides studying the sensitivity of the obtained enhancement curve on the pulse parameters, such as the pulse duration and peak amplitude, we suggest that experimentally realizable broadband chaotic light can also be used instead of the white noise to observe similar features. The underlying enhancement mechanism is analyzed in the frequency domain by computing a frequency-resolved atomic gain profile, as well as in the time domain by controlling the relative delay between the action of the laser pulse and noise. |
| -- |
| Author(s): Georgescu, I (Georgescu, Ionut); Saalmann, U (Saalmann, Ulf); Rost, JM (Rost, Jan M.) |
|
Title:
Clusters under strong vuv pulses: A quantum-classical hybrid description incorporating plasma effects |
| Source: PHYSICAL REVIEW A, 76 (4): Art. No. 043203 OCT 2007 |
| Abstract: The quantum-classical hybrid description of rare-gas clusters interacting with intense light pulses which we have developed is described in detail. Much emphasis is put on the treatment of screening electrons in the cluster which set the time scale for the evolution of the system and form the link between electrons strongly bound to ions and quasifree plasma electrons in the cluster. As an example, we discuss the dynamics of an Ar-147 cluster exposed to a short vuv laser pulse of 20 eV photon energy. |
| -- |
| Author(s): Beims, MW (Beims, M. W.); Manchein, C (Manchein, C.); Rost, JM (Rost, J. M.) |
|
Title:
Origin of chaos in soft interactions and signatures of nonergodicity |
| Source: PHYSICAL REVIEW E, 76 (5): Art. No. 056203 Part 2 NOV 2007 |
| Abstract: The emergence of chaotic motion is discussed for hard-point like and soft collisions between two particles in a one-dimensional box. It is known that ergodicity may be obtained in hard-point like collisions for specific mass ratios gamma=m(2)/m(1) of the two particles and that Lyapunov exponents are zero. However, if a Yukawa interaction between the particles is introduced, we show analytically that positive Lyapunov exponents are generated due to double collisions close to the walls. While the largest finite-time Lyapunov exponent changes smoothly with gamma, the number of occurrences of the most probable one, extracted from the distribution of finite-time Lyapunov exponents over initial conditions, reveals details about the phase-space dynamics. In particular, the influence of the integrable and pseudointegrable dynamics without Yukawa interaction for specific mass ratios can be clearly identified and demonstrates the sensitivity of the finite-time Lyapunov exponents as a phase-space probe. Being not restricted to two-dimensional problems such as Poincare sections, the number of occurrences of the most probable Lyapunov exponents suggests itself as a suitable tool to characterize phase-space dynamics in higher dimensions. This is shown for the problem of two interacting particles in a circular billiard. |
| -- |
| Author(s): Georgescu, I (Georgescu, Ionut); Saalmann, U (Saalmann, Ulf); Rost, JM (Rost, Jan M.) |
|
Title:
Attosecond resolved charging of ions in a rare-gas cluster |
| Source: PHYSICAL REVIEW LETTERS, 99 (18): Art. No. 183002 NOV 2 2007 |
| Abstract: A scheme to probe dissipative multielectron motion in time is introduced. In this context attosecond probing enables one to obtain information which is lost at later times and cannot be retrieved by conventional methods in the energy domain due to the incoherent nature of the dynamics. As a specific example we will trace the transient charging of ions in a rare-gas cluster during a strong femtosecond vacuum-ultraviolet pulse by means of delayed attosecond pulses. |
| -- |
| Author(s): Killian, TC (Killian, T. C.); Pattard, T (Pattard, T.); Pohl, T (Pohl, T.); Rost, JM (Rost, J. M.) |
|
Title:
Ultracold neutral plasmas |
| Source: PHYSICS REPORTS-REVIEW SECTION OF PHYSICS LETTERS, 449 (4-5): 77-130 SEP 2007 |
|
Abstract:
Ultracold neutral plasmas formed by photoionizing laser-cooled atoms near the ionization threshold have electron temperatures in the 1-1000 K range and ion temperatures from tens of millikelvin to a few Kelvin. They represent a new frontier in the study of neutral plasmas, which traditionally deals with much hotter systems, but they also blur the boundaries of plasma, atomic, condensed matter, and low temperature physics. Modeling these plasmas challenges computational techniques and theories of non-equilibrium systems, so the field has attracted great interest from the theoretical and computational physics communities. By varying laser intensities and wavelengths it is possible to accurately set the initial plasma density and energy, and charged-particle-detection and optical diagnostics allow precise measurements for comparison with theoretical predictions. Recent experiments using optical probes demonstrated that ions in the plasma equilibrate in a strongly coupled fluid phase. Strongly coupled plasmas, in which the electrical interaction energy between charged particles exceeds the average kinetic energy, reverse the traditional energy hierarchy underlying basic plasma concepts such as Debye screening and hydrodynamics. Equilibration in this regime is of particular interest because it involves the establishment of spatial correlations between particles, and it connects to the physics of the interiors of gas-giant planets and inertial confinement fusion devices. (C) 2007 Elsevier B.V. All rights reserved. |
| -- |
| Author(s): Ates, C (Ates, C.); Pohl, T (Pohl, T.); Pattard, T (Pattard, T.); Rost, JM (Rost, J. M.) |
|
Title:
Many-body theory of excitation dynamics in an ultracold Rydberg gas |
| Source: PHYSICAL REVIEW A, 76 (1): Art. No. 013413 JUL 2007 |
| Abstract: We develop a theoretical approach for the dynamics of Rydberg excitations in ultracold gases,with a realistically large number of atoms. We rely on the reduction of the single-atom Bloch equations to rate equations, which is possible under various experimentally relevant conditions. Here, we explicitly refer to a two-step excitation scheme. We discuss the conditions under which our approach is valid by comparing the results with the solution of the exact quantum master equation for two interacting atoms. Concerning the emergence of an excitation blockade in a Rydberg gas, our results are in qualitative agreement with experiment. Possible sources of quantitative discrepancy are carefully examined. Based on the two-step excitation scheme, we predict the occurrence of an antiblockade effect and propose possible ways to detect this excitation enhancement experimentally in an optical lattice, as well as in the gas phase. |
| -- |
| Author(s): Scully, SWJ (Scully, S. W. J.); Emmons, ED (Emmons, E. D.); Gharaibeh, MF (Gharaibeh, M. F.); Phaneuf, RA (Phaneuf, R. A.); Kilcoyne, ALD (Kilcoyne, A. L. D.); Schlachter, AS (Schlachter, A. S.); Schippers, S (Schippers, S.); Muller, A (Mueller, A.); Chakraborty, HS (Chakraborty, H. S.); Madjet, ME (Madjet, M. E.); Rost, JM (Rost, J. M.) |
|
Title:
Comment on "Photoexcitation of a volume plasmon in C-60 ions" - Reply |
| Source: PHYSICAL REVIEW LETTERS, 98 (17): Art. No. 179602 APR 27 2007 |
| -- |
| Author(s): Czasch, A (Czasch, A.); Schoffler, M (Schoeffler, M.); Hattass, M (Hattass, M.); Schossler, S (Schoessler, S.); Jahnke, T (Jahnke, T.); Weber, T (Weber, Th.); Staudte, A (Staudte, A.); Titze, J (Titze, J.); Voss, S (Voss, S.); Grisenti, RE (Grisenti, R. E.); Jagutzki, O (Jagutzki, O.); Schmidt, LP (Schmidt, L. Ph.); Schmidt-Bocking, H (Schmidt-Boecking, H.); Dorner, R (Doerner, R.); Rost, JM (Rost, J. M.); Schneider, T (Schneider, T.); Liu, CN (Liu, C.-N.); Bray, I (Bray, I.); Kheifets, AS (Kheifets, A. S.); Bartschat, K (Bartschat, K.) |
|
Title:
Partial photoionization cross sections and angular distributions for double excitation of Helium up to the N=13 threshold |
| Source: JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA, 156: XLV-XLV MAY 2007 |
| -- |
| Author(s): Singh, KP (Singh, Kamal P.); Rost, JM (Rost, Jan M.) |
|
Title:
Optimal stochastic enhancement of photoionization |
| Source: PHYSICAL REVIEW LETTERS, 98 (16): Art. No. 160201 APR 20 2007 |
| Abstract: The effect of noise on the nonlinear photoionization of an atom due to a femtosecond pulse is investigated in the framework of the stochastic Schrodinger equation. A modest amount of white noise results in an enhancement of the net ionization yield by several orders of magnitude, giving rise to a form of quantum stochastic resonance. We demonstrate that this effect is preserved if the white noise is replaced by broadband chaotic light. |
| -- |
| Author(s): Czasch, A (Czasch, A.); Schoffler, M (Schoeffler, M.); Hattass, M (Hattass, M.); Schossler, S (Schoessler, S.); Jahnke, T (Jahnke, T.); Weber, T (Weber, Th.); Staudte, A (Staudte, A.); Titze, J (Titze, J.); Wimmer, C (Wimmer, C.); Kammer, S (Kammer, S.); Weckenbrock, M (Weckenbrock, M.); Voss, S (Voss, S.); Grisenti, RE (Grisenti, R. E.); Jagutzki, O (Jagutzki, O.); Schmidt, LPH (Schmidt, L. Ph. H.); Schmidt-Bocking, H (Schmidt-Boecking, H.); Dorner, R (Doerner, R.); Rost, JM (Rost, J. -M.); Schneider, T (Schneider, T.); Liu, CN (Liu, Chien-Nan); Bray, I (Bray, I.); Kheifets, AS (Kheifets, A. S.); Bartschat, K (Bartschat, K.) |
|
Title:
Comment on "Partial photoionization cross sections and angular distributions for double excitation of helium up to the N=13 threshold" - Reply |
| Source: PHYSICAL REVIEW LETTERS, 98 (10): Art. No. 109302 MAR 9 2007 |
| -- |
| Author(s): Emmanouilidou, A (Emmanouilidou, Agapi); Rost, JM (Rost, Jan Michael) |
|
Title:
Attosecond-time-scale multielectron collisions in the Coulomb four-body problem: Traces in classical probability densities |
| Source: PHYSICAL REVIEW A, 75 (2): Art. No. 022712 FEB 2007 |
| Abstract: In the triple ionization of the Li ground state by single photon absorption the three electrons escape to the continuum mainly through two collision sequences with individual collisions separated by time intervals on the attosecond scale. We investigate the traces of these two collision sequences in the classical probability densities. We show that each collision sequence has characteristic phase space properties which distinguish it from the other. Classical probability densities are the closest analog to quantum mechanical densities allowing our results to be directly compared to quantum mechanical results. |
2006 |
| Author(s): Emmanouilidou, A (Emmanouilidou, Agapi); Rost, JM (Rost, Jan M.) |
|
Title:
The Coulomb four-body problem in a classical framework: triple photoionization of lithium |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 39 (20): 4037-4048 OCT 28 2006 |
| Abstract: Formulating a quasiclassical approach we determine the cross section for the complete four-body break-up of the lithium ground state following single photon absorption from threshold up to 220 eV excess energy. In addition, we develop a new classification scheme for three-electron ionizing trajectories in terms of electron-electron collisions, thereby identifying two main ionization paths which the three electrons in the ground state of lithium follow to escape to the continuum. The dominant escape paths manifest themselves in a characteristic 'T-shape' break-up pattern of the three electrons which implies observable structures in the electronic angular correlation probability. This break-up pattern prevails for excess energies so low that the Wannier threshold law sigma proportional to E-alpha describes already the triple ionization cross section, whose predicted value alpha = 2.16 we can confirm quantitatively. |
| -- |
| Author(s): Liu, ICH (Liu, I. C. H.); Rost, JM (Rost, J. M.) |
|
Title:
Polyatomic molecules formed with a Rydberg atom in an ultracold environment |
| Source: EUROPEAN PHYSICAL JOURNAL D, 40 (1): 65-71 OCT 2006 |
| Abstract: We investigate properties of ultralong-range polyatomic molecules formed with a Rb Rydberg atom and several ground-state atoms whose distance from the Rydberg atom is of the order of n(2)a(0), where n is the principle quantum number of the Rydberg electron. In particular, we put emphasis on the splitting of the energy levels, and elucidate the nature of the splitting via the construction of symmetry-adapted orbitals. |
| -- |
| Author(s): Ostrovsky, VN; Rost, JM |
|
Title:
Wannier-type threshold laws for multifragmentation and Thomson's problem |
| Source: FEW-BODY SYSTEMS, 38 (2-4): 153-160 JUN 2006 |
| Abstract: Wannier-type threshold theory for multiple break-up processes is based on the existence of special classical trajectories which represent partial fixed points of the equations of motion in a system of charged particles. These trajectories preserve the shape of the initial configuration while only changing its overall size in time. The relation between such scaling configurations and Thomson's (or surface Coulomb) problem is analyzed. In particular, it is shown that for eight electrons the twisted cube configuration solves Thomson's problem and also governs the threshold break-up of eight electrons receding from a charged core. The relevant exponents for the threshold power law are evaluated. |
| -- |
| Author(s): Ates, C; Pohl, T; Pattard, T; Rost, JM |
|
Title:
Strong interaction effects on the atom counting statistics of ultracold Rydberg gases |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 39 (11): L233-L239 JUN 14 2006 |
| Abstract: Based on simple rate equations for the Rydberg excitation process, we are able to model microscopically the dynamics of Rydberg excitation in ensembles of a large number of ultracold atoms, which is beyond the capabilities of fully ab initio approaches. Our results for the distribution of Rydberg atom numbers are in good agreement with recent experimental data, confirming the quenching of the distribution caused by Rydberg-Rydberg interactions. |
| -- |
| Author(s): Emmanouilidou, A; Rost, JM |
|
Title:
Triple photoionization of lithium near threshold |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 39 (5): L99-L103 MAR 14 2006 |
| Abstract: Solving the full classical four-body Coulomb problem numerically using a Wigner initial distribution, we formulate a classical-quantum hybrid approach to study triple ionization by single photon absorption from the Li ground state in the threshold region. We confirm the Wannier threshold law alpha proportional to E-alpha, and we show that a determined in the interval between 2 and 5 eV deviates from the analytical threshold value of 2.16 which we find in the interval between 0.1 and 2 eV. |
| -- |
| Author(s): Islam, MR; Saalmann, U; Rost, JM |
|
Title:
Kinetic energy of ions after Coulomb explosion of clusters induced by an intense laser pulse |
| Source: PHYSICAL REVIEW A, 73 (4): Art. No. 041201 APR 2006 |
| Abstract: It is shown that the kinetic-energy distribution of ions emerging from a cluster target irradiated by an intense laser pulse arises from three main effects: (1) the spatial profile of the laser beam, (2) the cluster size distribution in the experiment, and (3) possible saturation effects in the cluster ionization. Our model reveals that each of these effects leaves a characteristic fingerprint in the ion kinetic-energy spectrum. Moreover, it provides a quantitative link between observable ion spectra under experimental conditions and the ideal single-cluster result of a typical calculation. |
| -- |
| Author(s): Saalmann, U; Siedschlag, C; Rost, JM |
|
Title:
Mechanisms of cluster ionization in strong laser pulses |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 39 (4): R39-R77 FEB 28 2006 |
| Abstract: Femtosecond laser pulses have proven to provide valuable insight into the dynamics of microscopic systems by using pump-probe techniques. Applied to atomic clusters even a single pulse of varying pulse duration can reveal how and when energy from the laser pulse is transferred effectively to the cluster. We review the main experimental observables for energy transfer to a cluster and the major theoretical approaches which have been devised. Most importantly, we compare the cluster response to standard 780 nm light pulses with the response to 100 nm pulses, already obtained at a VUV free electron laser (FEL) source, and with 3 nm light which will be available from x-ray FEL sources. |
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| Author(s): Gianturco, FA; Franz, J; Buenker, RJ; Liebermann, HP; Pichl, L; Rost, JM; Tachikawa, M; Kimura, M |
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Title:
Positron binding to alkali-metal hydrides: The role of molecular vibrations |
| Source: PHYSICAL REVIEW A, 73 (2): Art. No. 022705 FEB 2006 |
| Abstract: The bound vibrational levels for J=0 have been computed for the series of alkali-metal hydride molecules from LiH to RbH, including NaH and KH. For all four molecules the corresponding potential-energy curves have been obtained for each isolated species and for its positron-bound complex (e(+)XH). It is found that the calculated positron affinity values strongly depend on the molecular vibrational state for which they are obtained and invariably increase as the molecular vibrational energy content increases. The consequences of our findings on the likelihood of possibly detecting such weakly bound species are briefly discussed. |
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| Author(s): Jiang, YH; Puttner, R; Hentges, R; Viefhaus, J; Poygin, M; Cacho, C; Becker, U; Rost, JM; Kaindl, G |
|
Title:
Partial photoionization cross sections of doubly excited helium below the ionization thresholds I-8 and I-9 |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 39 (1): L9-L16 JAN 14 2006 |
| Abstract: Partial photoionization cross sections (PCSS), a,, leading to final states of singly ionized helium, He+(n), were measured in the region of doubly excited helium below the ionization thresholds I-8 and I-9. The experiments were performed at BESSY II at high photon resolution, Delta E congruent to 6 meV, using a time-of-flight electron spectrometer. A comparison with recent eigenchannel R-matrix calculations reveals good agreement. The results of these measurements underline previous studies on quantum chaos in helium, which were mainly based on theoretical results. They also allow a critical assessment of the theoretical methods that produce the data used for a statistical analysis of double-excitation states with respect to quantum chaos, which is expected to occur very close to the double-ionization threshold. PCSs provide additional information to that derivable from total cross sections (TCSs). In the present PCS spectra, the resonance 8, 4(10) of the secondary series, which could not be resolved in the TCS, is clearly observed. |
2005 |
| Author(s): Pichl, L; Tachikawa, M; Buenker, RJ; Kimura, M; Rost, JM |
|
Title:
The effects of positron binding and annihilation mechanisms in biomolecules on PET resolution |
| Source: IEEE TRANSACTIONS ON NUCLEAR SCIENCE, 52 (6): 2810-2817 Part 2 DEC 2005 |
| Abstract: The angular correlation distribution of electron-positron annihilation photons is presented for polar biological molecules, and the associated noncollinearity effect on resolution in positron emission tomography (PET) is discussed. The image resolution in PET is known to be limited by a number of factors such as radioactive decay statistics, attenuation, scatter, or random coincidence of annihilation photons. This paper deals with the resolution limits on the side of the measured system. Our calculated distribution of annihilation momentum belongs to the intrinsic resolution factors together with the positron range in biological tissue; the other factors are due to the interactions along the photon path from the tissue to the detector. Here we demonstrate that the angular distribution of annihilation trajectories is not necessarily Gaussian, especially when the positron temporarily binds in a molecular dipole moment field before annihilation. The theoretical framework treats the electron and positron on the same footing by using the multireference single- and double-excitation configuration interaction algorithms applicable also to other bound-state systems. Our results form a subset of parameters relevant to the statistical image reconstruction algorithms, which may eventually increase the sensitivity of PET imaging in clinical setting. |
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| Author(s): Czasch, A; Schoffler, M; Hattass, M; Schossler, S; Jahnke, T; Weber, T; Staudte, A; Titze, J; Wimmer, C; Kammer, S; Weckenbrock, M; Voss, S; Grisenti, RE; Jagutzki, O; Schmidt, LPH; Schmidt-Bocking, H; Dorner, R; Rost, JM; Schneider, T; Liu, CN; Bray, I; Kheifets, AS; Bartschat, K |
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Title:
Partial photoionization cross sections and angular distributions for double excitation of helium up to the N=13 threshold |
| Source: PHYSICAL REVIEW LETTERS, 95 (24): Art. No. 243003 DEC 9 2005 |
| Abstract: Partial photoionization cross sections sigma(N)(E-gamma) and photoelectron angular distributions beta(N)(E-gamma) were measured for the final ionic states He+(N > 4) in the region between the N=8 and N=13 thresholds (E-gamma> 78.155 eV) using the cold target recoil ion momentum spectroscopy technique (COLTRIMS). Comparison of the experimental data with two independent sets of theoretical predictions reveals disagreement for the branching ratios to the various He-N(+) states. The angular distributions just below the double ionization threshold suggest an excitation process for highly excited N states similar to the Wannier mechanism for double ionization. |
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| Author(s): Kenfack, A; Rost, JM |
|
Title:
Stochastic dissociation of diatomic molecules |
| Source: JOURNAL OF CHEMICAL PHYSICS, 123 (20): Art. No. 204322 NOV 22 2005 |
| Abstract: The fragmentation of diatomic molecules under a stochastic force is investigated both classically and quantum mechanically, focusing on their dissociation probabilities. It is found that the quantum system is more robust than the classical one in the limit of a large number of kicks. The opposite behavior emerges for a small number of kicks. Quantum and classical dissociation probabilities do not coincide for any parameter combinations of the force. This can be attributed to a scaling property in the classical system which is broken quantum mechanically. (c) 2005 American Institute of Physics. |
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| Author(s): Saalmann, U; Rost, JM |
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Title:
Electron dynamics in strong laser pulse illumination of large rare gas clusters |
| Source: EUROPEAN PHYSICAL JOURNAL D, 36 (2): 159-164 NOV 2005 |
| Abstract: We analyze the dynamics of up to 10(5) electrons resulting from illuminating a xenon cluster with 9093 atoms with intense laser pulses of different length and peak intensity. Interesting details of electron motion are identified which can be probed with a time resolution of 100 attoseconds. Corresponding experiments would shed light on unexplored territory in complex electronic systems such as clusters and they would also permit to critically access the present theoretical description of this dynamics. |
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| Author(s): Pattard, T; Pohl, T; Rost, JM |
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Title:
Ultracold neutral plasmas and Rydberg gases |
| Source: NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 233: 132-140 MAY 2005 |
| Abstract: We present two different theoretical approaches for the simulation of ultracold plasmas and Rydberg gases, namely a simple kinetic model including an approximate treatment of ionic correlations on the one hand and a hybrid molecular dynamics (MD) approach treating the electrons as a fluid but ions and atoms on a full MD level on the other hand. The simple kinetic model can be used in situations where the number of particles is so large that it prohibits an MD simulation, and, maybe even more importantly, it gives additional insight into the dynamics beyond that possible on the basis of MD simulations by providing simple evolution equations for the macroscopic parameters describing the state of the system. On the other hand, the MD approach provides a powerful method for the study of spatially resolved quantities, and it permits the study of scenarios where the ions are so strongly coupled that Coulomb crystallization occurs, which cannot be described by the kinetic model. (c) 2005 Published by Elsevier B.V. |
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| Author(s): Pohl, T; Pattard, T; Rost, JM |
|
Title:
Relaxation to nonequilibrium in expanding ultracold neutral plasmas |
| Source: PHYSICAL REVIEW LETTERS, 94 (20): Art. No. 205003 MAY 27 2005 |
| Abstract: We investigate the strongly correlated ion dynamics and the degree of coupling achievable in the evolution of freely expanding ultracold neutral plasmas. We demonstrate that the ionic Coulomb coupling parameter Gamma(i) increases considerably in later stages of the expansion, reaching the strongly coupled regime despite the well known initial drop of Gamma(i) to order unity due to disorder-induced heating. Furthermore, we formulate a suitable measure of correlation and show that Gamma(i) calculated from the ionic temperature and density reflects the degree of order in the system if it is sufficiently close to a quasisteady state. At later times, however, the expansion of the plasma cloud becomes faster than the relaxation of correlations, and the system does not reach thermodynamic equilibrium anymore. |
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| Author(s): Siedschlag, C; Rost, JM |
|
Title:
Surface-plasma resonance in small rare-gas clusters by mixing ir and vuv laser pulses |
| Source: PHYSICAL REVIEW A, 71 (3): Art. No. 031401 Part A MAR 2005 |
| Abstract: The ionization dynamics of a xenon cluster with 40 atoms is analyzed under a pump-probe scenario of laser pulses where an infrared laser pulse of 50-fs length follows with a well-defined time delay a vuv pulse of the same length and peak intensity. The mechanism of resonant energy absorption due to the coincidence of the ir laser frequency with the frequency of collective motion of quasifree electrons in the cluster is mapped out by varying the time delay between the pulses. |
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| Author(s): Zimmermann, B; Lein, M; Rost, JM |
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Title:
Analysis of recombination in high-order harmonic generation in molecules |
| Source: PHYSICAL REVIEW A, 71 (3): Art. No. 033401 Part B MAR 2005 |
| Abstract: We show that the dependence of high-order harmonic generation (HHG) on the molecular orientation can be understood within a theoretical treatment that does not involve the strong field of the laser. The results for H-2 show excellent agreement with time-dependent strong-field calculations for model molecules, and this motivates a prediction for the orientation dependence of HHG from the N-2 3 sigma(g) valence orbital. For both molecules, we find that the polarization of recombination photons is influenced by the molecular orientation. The variations are particularly pronounced for the N-2 valence orbital, which can be explained by the presence of atomic p orbitals. |
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| Author(s): Scully, SWJ; Emmons, ED; Gharaibeh, MF; Phaneuf, RA; Kilcoyne, ALD; Schlachter, AS; Schippers, S; Muller, A; Chakraborty, HS; Madjet, ME; Rost, JM |
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Title:
Photoexcitation of a volume plasmon in C-60 ions |
| Source: PHYSICAL REVIEW LETTERS, 94 (6): Art. No. 065503 FEB 18 2005 |
| Abstract: Neutral C-60 is well known to exhibit a giant resonance in its photon absorption spectrum near 20 eV. This is associated with a surface plasmon, where delocalized electrons oscillate as a whole relative to the ionic cage. Absolute photoionization cross-section measurements for C-60(+), C-60(2+), and C-60(3+) ions in the 17-75 eV energy range show an additional resonance near 40 eV. Time-dependent density functional calculations confirm the collective nature of this feature, which is characterized as a dipole-excited volume plasmon made possible by the special fullerene geometry. |
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| Author(s): Dundas, D; Rost, JM |
|
Title:
Molecular effects in the ionization of N-2, O-2, and F-2 by intense laser fields |
| Source: PHYSICAL REVIEW A, 71 (1): Art. No. 013421 JAN 2005 |
| Abstract: In this paper we study the response in time of N-2, O-2, and F-2 to laser pulses having a wavelength of 390 nm. We find single-ionization suppression in O-2 and its absence in F-2, in accordance with experimental results at lambda = 800 nm. Within our framework of time-dependent density functional theory we are able to explain deviations from the predictions of intense-field many-body S-matrix theory (IMST). We confirm the connection of ionization suppression with destructive interference of outgoing electron waves from the ionized electron orbital. However, the prediction of ionization suppression, justified within the IMST approach through the symmetry of the highest occupied molecular orbital (HOMO), is not reliable since it turns out that-e.g., in the case of F-2-the electronic response to the laser pulse is rather complicated and does not lead to dominant depletion of the HOMO. Therefore, the symmetry of the HOMO is not sufficient to predict ionization suppression. However, at least for F-2, the symmetry of the dominantly ionized orbital is consistent with the nonsuppression of ionization. |
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| Author(s): Pohl, T; Pattard, T; Rost, JM |
|
Title:
Influence of electron-ion collisions on Coulomb crystallization of ultracold neutral plasmas |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 38 (2): S343-S350 Sp. Iss. SI JAN 28 2005 |
| Abstract: While ion heating by elastic electron-ion collisions may be neglected for a description of the evolution of freely expanding ultracold neutral plasmas, the situation is different in scenarios where the ions are laser-cooled during the system evolution. We show that electron-ion collisions in laser-cooled plasmas influence the ionic temperature, decreasing the degree of correlation obtainable in such systems. However, taking into account the collisions increases the ion temperature much less than what would be estimated based on static plasma clouds neglecting the plasma expansion. The latter leads to both adiabatic cooling of the ions as well as, more importantly, a rapid decrease of the collisional heating rate. |
2004 |
| Author(s): Pohl, T; Pattard, T; Rost, JM |
|
Title:
Kinetic modeling and molecular dynamics simulation of ultracold neutral plasmas including ionic correlations |
| Source: PHYSICAL REVIEW A, 70 (3): Art. No. 033416 SEP 2004 |
| Abstract: A kinetic approach for the evolution of ultracold neutral plasmas including interionic correlations and the treatment of ionization/excitation and recombination/deexcitation by rate equations is described in detail. To assess the reliability of the approximations inherent in the kinetic model, we have developed a hybrid molecular dynamics method. Comparison of the results reveals that the kinetic model describes the atomic and ionic observables of the ultracold plasma surprisingly well, confirming our earlier findings concerning the role of ion-ion correlations [Phys. Rev. A 68, 010703 (2003)]. In addition, the molecular dynamics approach allows one to study the relaxation of the ionic plasma component toward thermodynamical equilibrium. |
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| Author(s): Siedschlag, C; Rost, JM |
|
Title:
Small rare-gas clusters in soft x-ray pulses |
| Source: PHYSICAL REVIEW LETTERS, 93 (4): Art. No. 043402 JUL 23 2004 |
| Abstract: We develop a microscopic model for the interaction of small rare-gas clusters with soft x-ray radiation from a free electron laser. It is shown that, while the overall charging of the clusters is rather low, unexpectedly high atomic charge states can arise due to charge imbalances inside the cluster. These findings are explained by an increased absorption via inverse bremsstrahlung due to high intermediate charge states and by a nonhomogenous charge distribution inside the cluster. |
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| Author(s): Pattard, T; Pohl, T; Rost, JM |
|
Title:
Existence and stability of cold Rydberg gases and Plasmas |
| Source: FEW-BODY SYSTEMS, 34 (1-3): 181-184 MAY 2004 |
| Abstract: Recently, the spontaneous evolution of an ultracold gas of Rydberg atoms into a plasma has been demonstrated experimentally, as well as the reverse process of plasma recombination into atomic Rydberg states. Here, we discuss the existence and stability properties of the gas and plasma phases over a wide range of initial-state parameters. |
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| Author(s): Jiang, YH; Puttner, R; Martins, M; Follath, R; Rost, JM; Kaindl, G |
|
Title:
Isotope shifts of double-excitation resonances in helium |
| Source: PHYSICAL REVIEW A, 69 (5): Art. No. 052703 MAY 2004 |
| Abstract: Isotopic effects on double-excitation resonances in He-3 and He-4 were observed in photoionization spectra using synchrotron radiation with high monochromator resolution. In He-3, the resonances were found to be shifted to lower energies with respect to He-4 by DeltaE=3.06+/-0.07 meV, in good agreement with theoretical expectations based on normal and specific mass shifts. From the experimental data, the resonance parameters E-r, Gamma, and q of the resonances 2,-1(3), 2,1(4), and 2,0(7) were analyzed in detail using a double-convolution fit procedure that considers the different Doppler broadenings of the states in He-3 and He-4. In this way, the resolution function of the monochromator could be derived, which can be accounted for in a quantitative way on the basis of the properties of undulator radiation and the optical design of the monochromator. |
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| Author(s): Pohl, T; Pattard, T; Rost, JM |
|
Title:
On the possibility of 'correlation cooling' of ultracold neutral plasmas |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 37 (9): L183-L191 MAY 14 2004 |
| Abstract: Recent experiments with ultracold neutral plasmas show an intrinsic heating effect based on the development of spatial correlations. We investigate whether this effect can be reversed, so that imposing strong spatial correlations could in fact lead to cooling of the ions. We find that cooling is indeed possible. It requires, however, a very precise preparation of the initial state. Quantum mechanical zero-point motion sets a lower limit for ion cooling. |
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| Author(s): Kenfack, A; Rost, JM; de Almeida, AMO |
|
Title:
Optimal representations of quantum states by Gaussians in phase space |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 37 (8): 1645-1657 APR 28 2004 |
| Abstract: A two-step optimization is proposed to represent an arbitrary quantum state to the desired accuracy with the smallest number of Gaussians in phase space. The Husimi distribution of the quantum state provides the information to determine the modulus of the weight for the Gaussians. Then, the phase information contained in the Wigner distribution is used to obtain the full complex weights by considering the relative phases for pairs of Gaussians, the chords. The method is exemplified with excited states n of the harmonic and the Morse oscillators. A semiclassical interpretation of the number of Gaussians needed as a function of the quantum number n is given. |
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| Author(s): Jiang, YH; Puttner, R; Hentges, R; Viefhaus, J; Poiguine, M; Becker, U; Rost, JM; Kaindl, G |
|
Title:
Partial cross sections of doubly excited helium below the ionization threshold I-7 |
| Source: PHYSICAL REVIEW A, 69 (4): Art. No. 042706 APR 2004 |
| Abstract: Partial photoionization cross sections (PCSs), sigma(n), leading to final ionic states of helium, He+(n), were measured at BESSY II in the region of doubly excited helium up to the ionization threshold I-7 of He+. The experiments were performed with a time-of-flight (TOF) electron spectrometer and high photon resolution, DeltaEcongruent to6 meV. The results of these measurements are a most critical assessment of the decay dynamics of double-excitation resonances and agree well with those of recent eigenchannel R-matrix calculations. They also confirm the propensity rules set up for the autoionization of doubly excitated states. The mirroring behavior in the PCSs predicted recently by Liu and Starace is only partially observed. By discussing the formulas given by these authors in a more general context, the specific behavior of the PCSs of helium with respect to mirroring can be understood. The mirroring compensation properties between the "fractional partial cross sections" gamma(P)=sigma(P)/sigma(T) and gamma(Q)=sigma(Q)/sigma(T), with sigma(T)=sigma(P)+sigma(Q), are introduced and discussed. |
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| Author(s): Pohl, T; Pattard, T; Rost, JM |
|
Title:
Coulomb crystallization in expanding laser-cooled neutral plasmas |
| Source: PHYSICAL REVIEW LETTERS, 92 (15): Art. No. 155003 APR 16 2004 |
| Abstract: We present long-time simulations of expanding ultracold neutral plasmas, including a full treatment of the strongly coupled ion dynamics. Thereby, the relaxation of the expanding laser-cooled plasma is studied, taking into account elastic as well as inelastic collisions. It is demonstrated that, depending on the initial conditions, the ionic component of the plasma may exhibit short-range order or even a superimposed long-range order resulting in concentric ion shells. In contrast to ionic plasmas confined in traps, the shell structures build up from the center of the plasma cloud rather than from the periphery. |
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| Author(s): Harabati, C; Rost, JM; Grossmann, F |
|
Title:
Long-time and unitary properties of semiclassical initial value representations |
| Source: JOURNAL OF CHEMICAL PHYSICS, 120 (1): 26-30 JAN 1 2004 |
| Abstract: We numerically compare the semiclassical "frozen Gaussian" Herman-Kluk propagator [Chem. Phys. 91, 27 (1984)] and the "thawed Gaussian" propagator put forward recently by Baranger [J. Phys. A 34, 7227 (2001)] by studying the quantum dynamics in some nonlinear one-dimensional potentials. The reasons for the lack of long-time accuracy and norm conservation in the latter method are uncovered. We amend the thawed Gaussian propagator with a global harmonic approximation for the stability of the trajectories and demonstrate that this revised propagator is a true alternative to the Herman-Kluk propagator with similar accuracy. (C) 2004 American Institute of Physics. |
2003 |
| Author(s): Lein, M; Rost, JM |
|
Title:
Ultrahigh harmonics from laser-assisted ion-atom collisions |
| Source: PHYSICAL REVIEW LETTERS, 91 (24): Art. No. 243901 DEC 12 2003 |
| Abstract: We present a theoretical analysis of high-order harmonic generation from ion-atom collisions in the presence of linearly polarized intense laser pulses. Photons with frequencies significantly higher than in standard atomic high-harmonic generation are emitted. These harmonics are due to two different mechanisms: (i) collisional electron capture and subsequent laser-driven transfer of an electron between projectile and target atom; (ii) reflection of a laser-driven electron from the projectile leading to recombination at the parent atom. |
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| Author(s): Saalmann, U; Rost, JM |
|
Title:
Ionization of clusters in intense laser pulses through collective electron dynamics |
| Source: PHYSICAL REVIEW LETTERS, 91 (22): Art. No. 223401 NOV 28 2003 |
| Abstract: The motion of electrons and ions in medium-sized rare gas clusters (similar to1000 atoms) exposed to intense laser pulses is studied microscopically by means of classical molecular dynamics using a hierarchical tree code. Pulse parameters for optimum ionization are found to be wavelength dependent. This resonant behavior is traced back to a collective electron oscillation inside the charged cluster. It is shown that this dynamics can be well described by a driven and damped harmonic oscillator allowing for a clear discrimination against other energy absorption mechanisms. |
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| Author(s): Barna, IF; Rost, JM |
|
Title:
Photoionization of helium with ultrashort XUV laser pulses |
| Source: EUROPEAN PHYSICAL JOURNAL D, 27 (3): 287-290 DEC 2003 |
| Abstract: Coupled-channel calculations for multiphoton ionization probabilities of helium through interaction with intensive short laser pulses are presented. Besides Slater-like orbitals we use regular Coulomb wavepackets in our configurational interaction basis to describe the continuum. Linearly polarized laser pulses of 3.8 fs duration and 2.96 x 10(14) Wcm(-2) peak intensity have been used for frequencies between 0.2-1.2 a.u. The results are compared with other ab initio calculations. |
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| Author(s): Anding, K; Gross, P; Rost, JM; Allgaier, D; Jacobs, E |
|
Title:
The influence of uraemia and haemodialysis on neutrophil phagocytosis and antimicrobial killing |
| Source: NEPHROLOGY DIALYSIS TRANSPLANTATION, 18 (10): 2067-2073 OCT 2003 |
|
Abstract:
Background. Neutrophil functions in haemodialysis (HD) patients are altered by uraemia and by HD procedure. We investigated details of the neutrophil dysfunction as its nature and origin is not well understood. This is reflected by conflicting results about neutrophil phagocytosis activity and by scarce data on the neutrophil killing capability in HD patients. Methods. Using a flow-cytometric test system we have measured simultaneously phagocytosis and the production of reactive oxygen species (ROS) of neutrophils and in parallel antimicrobial killing of yeast by neutrophils. 117 whole-blood samples of healthy controls and 50 pre- and 50 post-dialysis samples of HD patients, half of them with diabetes mellitus (DM), have been evaluated. We have constructed a model to account for the dependence on the stimulus-to-cell ratio and obtain means for phagocytosis and killing at different incubation times. Results. (i) HD patients have significantly lower neutrophil killing (20%) than healthy controls. (ii) Dialysis improves the killing capability by 10-15%, after dialysis the killing activity remains significantly (10%) below that of the controls. (iii) The percentage of neutrophils, which exhibit phagocytosis and produce ROS. does not differ significantly between HD patients and healthy controls. (iv) Age has no significant influence on phagocytosis and killing. Conclusion. The neutrophil killing capability is reduced in HD patients while the amount of neutrophils that phagocyte and produce ROS remains unchanged. Functional impairment of uraemic neutrophils is therefore mainly a result of their reduced capability to kill microorganisms intracellularly. |
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| Author(s): Brand, J; Haring, I; Rost, JM |
|
Title:
Levinson-like theorem for scattering from a Bose-Einstein condensate |
| Source: PHYSICAL REVIEW LETTERS, 91 (7): Art. No. 070403 AUG 15 2003 |
| Abstract: A relation between the number of bound elementary excitations of an atomic Bose-Einstein condensate and the phase shift of elastically scattered atoms is derived. Within the Bogoliubov model of a weakly interacting Bose gas this relation is exact and generalizes Levinson's theorem. Specific features of the Bogoliubov model such as complex energy and continuum bound states are discussed and a numerical example is given. |
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| Author(s): Pohl, T; Pattard, T; Rost, JM |
|
Title:
Plasma formation from ultracold Rydberg gases |
| Source: PHYSICAL REVIEW A, 68 (1): Art. No. 010703 JUL 2003 |
| Abstract: Recent experiments have demonstrated the spontaneous evolution of a gas of ultracold Rydberg atoms into an expanding ultracold plasma, as well as the reverse process of plasma recombination into highly excited atomic states. Treating the evolution of the plasma on the basis of kinetic equations, while ionization/excitation and recombination are incorporated using rate equations, we have investigated theoretically the Rydberg-to-plasma transition. Including the influence of spatial correlations on the plasma dynamics in an approximate way, we find that ionic correlations change the results quantitatively but not qualitatively. |
| -- |
| Author(s): Emmanouilidou, A; Schneider, T; Rost, JM |
|
Title:
Quasiclassical double photoionization from the 2 S-1,S-3 excited states of helium including shake-off |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 36 (13): 2717-2724 JUL 14 2003 |
| Abstract: We account for the different symmetries of the 2 S-1,S-3 helium excited states in a quasiclassical description of the knockout mechanism, augmented by a quantum shake-off contribution. We are thus able to formulate the separate contributions of the knockout and shake-off mechanisms for double photoionization for any. excess energy from the 2 S-1,S-3 states. Photoionization ratios and single-differential cross sections calculated for the 2 S-1,S-3 excited states of helium are found to be in very good agreement with recent theoretical results. |
| -- |
| Author(s): Pattard, T; Schneider, T; Rost, JM |
|
Title:
On the role of shake-off in single-photon double ionization |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 36 (12): L189-L195 JUN 28 2003 |
| Abstract: The role of shake-off for double ionization of atoms by a single photon with finite energy has become the subject of debate. In this letter, we attempt to clarify the meaning of shake-off at low photon energies by comparing different formulations appearing in the literature and by suggesting a working definition. Moreover, we elaborate on the foundation and justification of a mixed quantum-classical ansatz for the calculation of single-photon double ionization. |
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| Author(s): Geyer, T; Rost, JM |
|
Title:
Dynamical stabilization of classical multi-electron targets against autoionization |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 36 (4): L107-L112 FEB 28 2003 |
| Abstract: We demonstrate that a recently published quasiclassical Moller type approach (Geyer and Rost 2002 J. Phys. B: At. Mol. Opt. Phys. 35 1479) can be used to overcome the problem of autoionization, which arises in classical trajectory calculations for many-electron targets. In this method, the target is stabilized dynamically by a backward-forward propagation scheme. We illustrate this refocusing and present total cross sections for single and double ionization of helium by electron impact. |
| -- |
| Author(s): Schneider, T; Rost, JM |
|
Title:
Double photoionization of two-electron atoms based on the explicit separation of dominant ionization mechanisms |
| Source: PHYSICAL REVIEW A, 67 (6): Art. No. 062704 JUN 2003 |
| Abstract: Double ionization by a single photon is often discussed in terms of two mechanisms, namely, shakeoff and knockout, dominant at high and low photon energies, respectively. We have developed a model to formulate the explicit but separate contribution of both mechanisms at all energies [Phys. Rev. Lett. 89, 073002 (2002)]. The separation is based on a quasiclassical formulation of knockout which is free from any shakeoff part since the latter is purely quantum mechanical. The relevance of each mechanism from threshold up to several keV photon energy is quantified and discussed in detail for the photoionization from the ground state of two-electron atoms. Photoionization ratios, integral and singly differential cross sections calculated for helium and other members of its isoelectronic sequence are compared to benchmark experimental data and recent theoretical results. A connection to Samson's half-collision model [Phys. Rev. Lett. 65, 2861 (1990)] is also given. |
| -- |
| Author(s): Anding, K; Rost, JM; Jacobs, E; Daschner, FD |
|
Title:
Flow cytometric measurements of neutrophil functions: the dependence on the stimulus to cell ratio |
| Source: FEMS IMMUNOLOGY AND MEDICAL MICROBIOLOGY, 35 (2): 147-152 MAR 20 2003 |
| Abstract: Phagocytosis and antimicrobial killing of neutrophils has been quantitatively determined as a function of the stimulus (Candida albicans) to cell ratio R using two donor collectives containing a total of 115 blood samples. Analysis of the collectives in two different laboratories according to the same flow cytometric protocol for simultaneous measurement of neutrophil functions did not produce statistically significant differences. The number of phagocytosing leukocytes as well as that of killed fungi per leukocyte depends strongly on R. While each phagocytosing neutrophil kills one fungus at low values of R, each neutrophil kills on average 2.5 fungi for large R. (C) 2003 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved. |
| -- |
| Author(s): Siedschlag, C; Rost, JM |
|
Title:
Enhanced ionization in small rare-gas clusters |
| Source: PHYSICAL REVIEW A, 67 (1): Art. No. 013404 JAN 2003 |
| Abstract: A detailed theoretical investigation of rare-gas atom clusters under intense short laser pulses reveals that the mechanism of energy absorption is akin to enhanced ionization first discovered for diatomic molecules. The phenomenon is robust under changes of the atomic element (neon, argon, krypton, xenon), the number of atoms in the cluster (16-30 atoms have been studied), and the fluence of the laser pulse. In contrast to molecules it does not disappear for circular polarization. We develop an analytical model relating the pulse length for maximum ionization to characteristic parameters of the cluster. |
2002 |
| Author(s): Siedschlag, C; Rost, JM |
|
Title:
Electron release of rare-gas atomic clusters under an intense laser pulse |
| Source: PHYSICAL REVIEW LETTERS, 89 (17): Art. No. 173401 OCT 21 2002 |
| Abstract: Calculating the energy absorption of atomic clusters as a function of the laser pulse length T we find a maximum for a critical T*. We show that T* can be linked to an optimal cluster radius R*. The existence of this radius can be attributed to the enhanced ionization mechanism originally discovered for diatomic molecules. Our findings indicate that enhanced ionization should be operative for a wide class of rare-gas clusters. From a simple Coulomb-explosion ansatz, we derive an analytical expression relating the maximum energy release to a suitably scaled expansion time which can be expressed with the pulse length T*. |
| -- |
| Author(s): Rudel, A; Hentges, R; Becker, U; Chakraborty, HS; Madjet, ME; Rost, JM |
|
Title:
Imaging delocalized electron clouds: Photoionization of C-60 in Fourier reciprocal space |
| Source: PHYSICAL REVIEW LETTERS, 89 (12): Art. No. 125503 SEP 16 2002 |
| Abstract: The dynamics of the photoionization of the two outermost orbitals of C-60 has been studied in the oscillatory regime from threshold to the carbon K edge. We show that geometrical properties of the fullerene electronic hull, such as its diameter and thickness, are contained in the partial photoionization cross sections by examining ratios of partial cross sections as a function of the photon wave number in the Fourier conjugated space. Evaluated in this unconventional manner pbotoemission data reveal directly the desired spatial information. |
| -- |
| Author(s): Saalmann, U; Rost, JM |
|
Title:
Ionization of clusters in strong x-ray laser pulses |
| Source: PHYSICAL REVIEW LETTERS, 89 (14): Art. No. 143401 SEP 30 2002 |
| Abstract: The effect of intense x-ray laser interaction on argon clusters is studied theoretically with a mixed quantum/classical approach. In comparison to a single atom we find that ionization of the cluster is suppressed, which is in striking contrast to the observed behavior of rare-gas clusters in intense optical laser pulses. We have identified two effects responsible for this phenomenon: A high space charge of the cluster in combination with a small quiver amplitude and delocalization of electrons in the cluster. We elucidate their impact for different field strengths and cluster sizes. |
| -- |
| Author(s): Geyer, T; Rost, JM |
|
Title:
A quasiclassical approach to electron impact ionization |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 35 (6): 1479-1499 MAR 28 2002 |
| Abstract: A quasiclassical approximation to quantum mechanical scattering in the Moller formalism is developed. While keeping the numerical advantage of a standard classical trajectory Monte Carlo calculation, our approach is no longer restricted to using stationary initial distributions. This allows one to improve the results by using better suited initial phase space distributions than the microcanonical one and to gain insight into the collision mechanism by studying the influence of different initial distributions on the cross section. A comprehensive account of results for single, double and triple differential cross sections for atomic hydrogen will be given, in comparison with experiment and other theories. |
| -- |
| Author(s): Schneider, T; Chocian, PL; Rost, JM |
|
Title:
Separation and identification of dominant mechanisms in double photoionization |
| Source: PHYSICAL REVIEW LETTERS, 89 (7): Art. No. 073002 AUG 12 2002 |
| Abstract: Double photoionization by a single photon is often discussed in terms of two contributing mechanisms, knockout (two-step-one) and shakeoff, with the latter being a pure quantum effect. It is shown that a quasiclassical description of knockout and a simple quantum calculation of shakeoff provides a clear separation of the mechanisms and facilitates their calculation considerably. The relevance of each mechanism at different photon energies is quantified for helium. Photoionization ratios, integral, and singly differential cross sections obtained by us are in excellent agreement with benchmark experimental data and recent theoretical results. |
| -- |
| Author(s): Siedschlag, C; Rost, JM |
|
Title:
Fragmentation in intense time-dependent fields |
| Source: FEW-BODY SYSTEMS, 31 (2-4): 211-216 2002 |
| Abstract: The mechanism of enhanced ionization or charge-assisted tunneling, first discovered for small linear molecules under an intense laser pulse, is briefly reviewed. Then it is shown that this mechanism also applies to rare gas clusters in strong laser fields. A suitable observable which can be accessed experimentally, is the degree of ionization of the cluster as a function of the laser pulse length. Examples are given for Ne-16 and Ar-16. |
| -- |
| Author(s): Nazmitdinov, RG; Simonovic, NS; Rost, JM |
|
Title:
Semiclassical analysis of a two-electron quantum dot in a magnetic field: Dimensional phenomena |
| Source: PHYSICAL REVIEW B, 65 (15): Art. No. 155307 APR 15 2002 |
| Abstract: It is shown that with the inclusion of the vertical extension of a quantum dot the experimental findings of Ashoori [Phys. Rev. Lett. 71, 613 (1993)] can be modeled consistently with a parabolic confinement. Furthermore, the magnetic properties such as the magnetic moment and the susceptibility are sensitive to the presence and strength of a vertical confinement. Using a semiclassical approach the calculation of the eigenvalues reduces to simple quadratures providing a transparent and almost analytical quantization of the three-dimensional quantum dot energy levels that differ from the exact energies only by a few percent. While the dynamics for three-dimensional axially symmetric two-electron quantum dot with parabolic confinement potentials is in general nonseparable due to the Coulomb interaction we have found an exact separability for specific values of the magnetic field. |
| -- |
| Author(s): Schneider, T; Liu, CN; Rost, JM |
|
Title:
Intermanifold similarities in partial photoionization cross sections of helium |
| Source: PHYSICAL REVIEW A, 65 (4): Art. No. 042715 Part A APR 2002 |
| Abstract: Using the eigenchannel R-matrix method we calculate partial photoionization cross sections from the ground state of the helium atom for incident photon energies up to the N=9 manifold. The wide energy range covered by our calculations permits a thorough investigation of general patterns in the cross sections which were first discussed by Menzel and coworkers [Phys. Rev. A 54, 2080 (1996)]. The existence of these patterns can easily be understood in terms of propensity rules for autoionization. As the photon energy is increased the regular patterns are locally interrupted by perturber states until they fade out indicating the progressive breakdown of the propensity rules and the underlying approximate quantum numbers. We demonstrate that the destructive influence of isolated perturbers can be compensated with an energy-dependent quantum defect. |
| -- |
| Author(s): Faria, CFD; Kopold, R; Becker, W; Rost, JM |
|
Title:
Resonant enhancements of high-order harmonic generation |
| Source: PHYSICAL REVIEW A, 65 (2): Art. No. 023404 FEB 2002 |
| Abstract: Solving the one-dimensional time-dependent Schrodinger equation for simple model potentials, we investigate resonance-enhanced high-order harmonic generation, with emphasis on the physical mechanism of the enhancement. By truncating a long-range potential, we investigate the significance of the long-range tail, the Rydbergg series, and the existence of highly excited states for the enhancements in question. We conclude that the channel closings typical of a short-range or zero-range potential are capable of generating essentially the same effects. |
2001 |
| Author(s): Simonovic, NS; Rost, JM |
|
Title:
The positronium negative ion: Classical properties and semiclassical quantization |
| Source: EUROPEAN PHYSICAL JOURNAL D, 15 (2): 155-164 AUG 2001 |
| Abstract: Properties of collinear and planar periodic orbits for the positronium negative ion are examined with respect to the possibilities for semiclassical quantization. In contrast to other two-electron atomic systems as helium and H- the relevant orbits for quantization are fully stable and permit a full torus quantization. However, for lower excitations the area of stability in phase-space is too small for a reliable torus quantization. Instead, a quasi-separability of the three-body system is used to apply effective one-dimensional (WKB) quantization. |
| -- |
| Author(s): Briggs, JS; Rost, JM |
|
Title:
On the derivation of the time-dependent equation of Schrodinger |
| Source: FOUNDATIONS OF PHYSICS, 31 (4): 693-712 APR 2001 |
| Abstract: Few have done more than Martin Gutzwiller to clarify the connection between classical time-dependent motion and the time-independent states of quantum systems. Hence it seems appropriate to include the following discussion of the origins of the time-dependent Schrodinger equation in this volume dedicated to him. |
| -- |
| Author(s): Madjet, ME; Chakraborty, HS; Rost, JM |
|
Title:
Spurious oscillations from local self-interaction correction in high-energy photoionization calculations for metal clusters |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 34 (10): L345-L351 MAY 28 2001 |
| Abstract: We find that for simple metal clusters a single-electron description of the ground state employing self-interaction correction (SIC) in the framework of local-density approximation strongly contaminates the high-energy photoionization cross sections with spurious oscillations for a subshell containing node(s). This effect is shown connected to the unphysical structure that SIC generates in ensuing state-dependent radial potentials around a position where the respective orbital density attains nodal zero. Non-local Hartree-Fock that exactly eliminates the electron self-interaction is found entirely free from this effect. It is inferred that while SIC is largely unimportant in high photon energies, any implementation of it within the local frame can induce unphysical oscillations in the high-energy photospectra of metal clusters pointing to a general need for caution in choosing appropriate theoretical tools. |
| -- |
| Author(s): Pattard, T; Rost, JM |
|
Title:
Threshold fragmentation under dipole forces |
| Source: FOUNDATIONS OF PHYSICS, 31 (3): 535-541 MAR 2001 |
| Abstract: The threshold law for N-body fragmentation under dipole forces is formulated. It emerges from the energy dependence of the normalization of the correlated continuum wave function for N fragments. It is shown that the dipole threshold law plats a key role in understanding all threshold fragmentation phenomena since it links the classical threshold law for long-range Coulomb interactions to the statistical law for short-range interactions. Futhermore, a tunnelling mechanism is identified as the common feature which occurs for all three classes of interactions, short-range, dipole and Coulomb. |
| -- |
| Author(s): Geyer, T; Rost, JM |
|
Title:
A quasi-classical approach to fully differential ionization cross sections |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 34 (2): L47-L53 JAN 28 2001 |
| Abstract: classical approximation to time-dependent quantum mechanical scattering in the Moller formalism is presented. Numerically, our approach is similar to a standard classical-trajectory Monte Carlo calculation. Conceptually, however, our formulation allows one to release the restriction to stationary initial distributions. This is achieved by a classical forward-backward propagation technique. As a first application and for comparison with experiment we present fully differential cross sections for electron-impact ionization of atomic hydrogen in the Erhardt geometry. |
| -- |
| Author(s): Rost, JM |
|
Title:
Critical phenomena in atomic physics |
| Source: PHYSICA E, 9 (3): 467-473 MAR 2001 |
| Abstract: It is shown that the threshold energy epsilon = 0 for complete break-up of N charged particles represents a fixed point of the dynamical system. Renormalization theory akin to thermodynamical phase transitions is used to determine the exponent beta of the power law for the fragmentation cross section sigma(epsilon) proportional to epsilon (beta) near the fixed point. In the generic case beta is the ratio of two rate constants expressed in terms of Liapunov exponents which emerge from the stability analysis of the critical point. Using the derived low and the well-known high-energy behavior, a universal shape function for direct ionization by charged particles is given. (C) 2001 Elsevier Science B.V. All rights reserved. |
2000 |
| Author(s): Pattard, T; Rost, JM |
|
Title:
Comment on "Degenerate Wannier theory for multiple ionization" - Pattard and Rost reply |
| Source: PHYSICAL REVIEW LETTERS, 85 (20): 4410-4410 NOV 13 2000 |
| -- |
| Author(s): Faria, CFD; Rost, JM |
|
Title:
High-order harmonic generation from a confined atom |
| Source: PHYSICAL REVIEW A, 62 (5): Art. No. 051402 NOV 2000 |
| Abstract: The order of high harmonics emitted by an atom in an intense laser field is limited by the so-called cutoff frequency. Solving the time-dependent Schrodinger equation, we show that this frequency can be increased considerably by a parabolic confining potential, if the confinement parameters are suitably chosen. Furthermore, due to confinement, the radiation intensity remains high throughout the extended emission range. All features observed can be explained with classical arguments. |
| -- |
| Author(s): van de Sand, G; Rost, JM |
|
Title:
Semiclassical description of multiphoton processes |
| Source: PHYSICAL REVIEW A, 62 (5): Art. No. 053403 NOV 2000 |
| Abstract: We analyze strong held atomic dynamics semiclassically, based on a fun time-dependent description with the Hermann-Kluk propagator. From the properties of the exact classical trajectories, in particular the accumulation of action in time, the prominent features of above-threshold ionization and higher-harmonic generation are proven to be interference phenomena. They are reproduced quantitatively in the semiclassical approximation. Moreover, the behavior of the action of the classical trajectories supports the so called strong field approximation which has been devised and postulated for strong field dynamics. |
| -- |
| Author(s): Briggs, JS; Rost, JM |
|
Title:
Time dependence in quantum mechanics |
| Source: EUROPEAN PHYSICAL JOURNAL D, 10 (3): 311-318 JUN 2000 |
| Abstract: It is shown that the time-dependent equations (Schrodinger and Dirac) for a quantum system can be derived from the time-independent equation for the larger object of the system interacting with its environment, in the limit that the dynamical variables of the environment can be treated semiclassically. The time which describes the quantum evolution is then provided parametrically by the classical evolution of the environment variables. The method used is a generalization of that known for a long time in the field of ion-atom collisions, where it appears as a transition from the full quantum mechanical perturbed stationary states to the impact parameter method in which the projectile ion beam is treated classically. |
| -- |
| Author(s): Tanner, G; Richter, K; Rost, JM |
|
Title:
The theory of two-electron atoms: between ground state and complete fragmentation |
| Source: REVIEWS OF MODERN PHYSICS, 72 (2): 497-544 APR 2000 |
| Abstract: Since the first attempts to calculate the helium ground state in the early days of Bohr-Sommerfeld quantization, two-electron atoms have posed a series of unexpected challenges to theoretical physics. Despite the seemingly simple problem of three charged particles with known interactions, it took more than half a century after quantum mechanics was established to describe the spectra of two-electron atoms satisfactorily. The evolution of the understanding of correlated two-electron dynamics and its importance for doubly excited resonance states is presented here, with an emphasis on the concepts introduced. The authors begin by reviewing the historical development and summarizing the progress in measuring the spectra of two-electron atoms and in calculating them by solving the corresponding Schrodinger equation numerically. They devote the second part of the review to approximate quantum methods, in particular adiabatic and group-theoretical approaches. These methods explain and predict the striking regularities of two-electron resonance spectra, including propensity rules for decay and dipole transitions of resonant states. This progress was made possible through the identification of approximate dynamical symmetries leading to corresponding collective quantum numbers for correlated electron-pair dynamics. The quantum numbers are very different from the independent particle classification, suitable for low-lying states in atomic systems. The third section of the review describes modern semiclassical concepts and their application to two-electron atoms. Simple interpretations of the approximate quantum numbers and propensity rules can be given in terms of a few key periodic orbits of the classical three-body problem. This includes the puzzling existence of Rydberg series for electron-pair motion. Qualitative and quantitative semiclassical estimates for doubly excited states are obtained for both regular and chaotic classical two-electron dynamics using modern semiclassical techniques. These techniques set the stage for a theoretical investigation of the regime of extreme excitation towards the three-body breakup threshold. Together with periodic orbit spectroscopy, they supply new tools for the analysis of complex experimental spectra. |
| -- |
| Author(s): van de Sand, G; Rost, JM |
|
Title:
Inelastic semiclassical Coulomb scattering |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 33 (7): 1423-1435 APR 14 2000 |
| Abstract: We present a semiclassical S-matrix study of inelastic collinear electron-hydrogen scattering. A simple way to extract all the necessary information from the deflection function alone without having to compute the stability matrix is described. This includes the determination of the relevant Maslov indices. Results of singlet and triplet cross sections for excitation and ionization are reported. The different levels of approximation-classical, semiclassical and uniform semiclassical-are compared among each other and with the full quantum result. |
1999 |
| Author(s): Rost, JM; Pattard, T |
|
Title:
Universal shape function for the double-ionization cross section of negative ions by electron impact |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 32 (15): L457-L460 AUG 14 1999 |
| Abstract: It is shown that recently measured cross sections for double ionization of negative ions (H-, O- and C-) possess a universal shape when plotted in suitable dimensionless units. The shape can be represented with a simple analytical function, following the same principles as those which have been utilized during the establishment of a universal shape function for single ionization (Rost and Pattard 1997 Phys. Rev. A 55 R5). Thereby, it is demonstrated that direct double ionization dominates the cross section for the targets considered. |
| -- |
| Author(s): Pattard, T; Rost, JM |
|
Title:
Cross sections for (multiple) ionization of ions in collisions with electrons |
| Source: PHYSICA SCRIPTA, T80B: 295-297 1999 |
| Abstract: Recently we proposed a new semiempirical analytical parametrization for the shape of ionization cross sections resulting from collisions of ions with various projectiles [1]. It was shown that the incorporation of the correct threshold behaviour as given by the Wannier theory is the crucial point to obtain good agreement with experimental results from threshold to high energies and leads to a unified view of collision processes in very different systems. While the shape of the cross sections as given by this parametrization already compares favourably with the experiment, a simple extension of the formula leads to an even more accurate description for electron-impact ionization of ions [2]. Using recent results from an extended Wannier theory for multiple ionization [3,4], tests of similar parametrizations for multiple ionization processes become feasible. |
| -- |
| Author(s): van de Sand, G; Rost, JM |
|
Title:
Irregular orbits generate higher harmonics |
| Source: PHYSICAL REVIEW LETTERS, 83 (3): 524-527 JUL 19 1999 |
| Abstract: The spectrum of higher harmonics in atoms calculated with a uniformized semiclassical propagator is presented and it is shown that the higher harmonic generation is an interference phenomenon which can be described semiclassically. This can be concluded from the good agreement with the quantum spectrum. Moreover, the formation of a plateau in the spectrum is specifically due to the interference of irregular, time-delayed, trajectories with regular orbits without a time delay. This is proven by the absence of the plateau in an artificial semiclassical spectrum generated from a sample of trajectories from which the irregular trajectories (only a few percent) have been discarded. |
| -- |
| Author(s): Frank, O; Rost, JM |
|
Title:
From collectivity to the single-particle picture in the photoionization of clusters |
| Source: PHYSICAL REVIEW A, 60 (1): 392-397 JUL 1999 |
| Abstract: Photoionization of alkali-metal clusters is investigated theoretically for different photon energy regimes. At low energies the photo cross section is characterized by the well-known plasmon peak resulting from collective electron dynamics. For high energies the ionization cross section exhibits an oscillatory behavior, which can be explained by single-particle effects. In this paper we use the random phase approximation (RPA) to calculate the photo cross section on an equal footing for both, the low- and the high-energy regime, Thereby, we can show that the cross sections for photoionization calculated in the collective RPA and in the single-particle picture indeed merge for high photon energies. Moreover, we demonstrate that the oscillatory behavior can already be identified at low photon energies where the cross section is not yet exponentially small. Hence, it should be possible to identify the oscillations experimentally. [S1059-2947(99)07706-9]. |
| -- |
| Author(s): van de Sand, G; Rost, JM |
|
Title:
Semiclassical time-dependent propagation in three dimensions for a Coulomb potential |
| Source: PHYSICAL REVIEW A, 59 (3): R1723-R1726 MAR 1999 |
| Abstract: A unified semiclassical time propagator is used to calculate the semiclassical rime-correlation function in three Cartesian dimensions for a particle moving in an attractive Coulomb potential. It is demonstrated that under these conditions the singularity of the potential does not cause any difficulties and the Coulomb interaction can be treated as any other nonsingular potential. Moreover, by virtue of our three-dimensional calculation, we can explain the discrepancies between previous semiclassical and quantum results obtained for the one-dimensional radial Coulomb problem. [S1050-2947(99)50303-X]. |
1998 |
| Author(s): Pattard, T; Rost, JM |
|
Title:
A route to chaos in classical atom-diatom collisions |
| Source: CHEMICAL PHYSICS LETTERS, 291 (3-4): 360-366 JUL 17 1998 |
| Abstract: We study classical atom-diatom collisions in the 'T-shaped' configuration. Considering collisions of helium atoms with a bound I, molecule at high energies, the scattering process becomes increasingly complicated as the projectile energy is lowered. The complexity manifests itself in phase space structures which change in a well defined way forming a 'route to chaos'. An analysis of this route provides a deeper understanding of the appearance of chaos in three-body collisional systems. Our perspective is complementary to approaches dealing directly with the fractal properties of observables in the chaotic regime. (C) 1998 Elsevier Science B.V. All rights reserved. |
| -- |
| Author(s): Aichele, K; Hartenfeller, U; Hathiramani, D; Hofmann, G; Schafer, V; Steidl, M; Stenke, M; Salzborn, E; Pattard, T; Rost, JM |
|
Title:
Electron impact ionization of the hydrogen-like ions B4+, C5+, N6+ and O7+ |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 31 (10): 2369-2379 MAY 28 1998 |
| Abstract: For the first time absolute cross sections for electron impact ionization of the hydrogen-like ions B4+, C5+, N6+ and O7+ at electron energies from below threshold up to about 6 keV have been measured using the crossed-beams technique. All measured cross sections are in very good agreement with available distorted-wave exchange calculations and the semiempirical Lotz formula. The behaviour of the classically scaled cross sections along the hydrogen isoelectronic sequence is discussed. A recently developed scaling technique allows us to predict all non-relativistic cross sections along the isoelectronic sequence on an absolute scale. |
| -- |
| Author(s): Pattard, T; Rost, JM |
|
Title:
Degenerate Wannier theory for multiple ionization |
| Source: PHYSICAL REVIEW LETTERS, 80 (23): 5081-5084 JUN 8 1998 |
| Abstract: It is shown that the cross section for multiple breakup of a system into charged fragments near the threshold energy epsilon = 0 follows a power law modified by logarithmic correction terms if the system possesses degenerate normal mode frequencies about the fixed point of the equilibrium configuration. For more than two identical particles, e.g., a multielectron atom, this will be the generic case since the equilibrium configuration is highly symmetric. The modified threshold law is derived using consistently the properties of the classical monodromy matrix about the fixed point to formulate the threshold cross section. |
| -- |
| Author(s): Rost, JM |
|
Title:
Semiclassical s-matrix theory for atomic fragmentation |
| Source: PHYSICS REPORTS-REVIEW SECTION OF PHYSICS LETTERS, 297 (6): 272-344 APR 1998 |
| Abstract: A semiclassical scattering approach is developed which can handle long-range (Coulomb) forces without the knowledge of thr asymptotic wave function for multiple charged fragments in the continuum. The classical cross section for potential and inelastic scattering including fragmentation (ionization) is derived from first principles in a form which allows for a simple extension to semiclassical scattering amplitudes as a stun over classical orbits and their associated actions, The object of primary importance is the classical deflection function which can show regular and chaotic behavior. Applications to electron impact ionization of hydrogen and electron-atom scattering in general are discussed in a reduced phase space, motivated by partial fixed points of the respective scattering systems, Special emphasis, also in connection with chaotic scattering, is put on threshold ionization, Finally, motivated by the reflection principle for molecules, a semiclassical hybrid approach is introduced for photoabsorption cross sections of atoms where the time-dependent propagator is approximated semiclassically in a short-time limit with the Baker-Hausdorff formula. Applications to one- and two-electron atoms are followed by a presentation of double photoionization of helium, treated in combination with :he semiclassical S-matrix for scattering. (C) 1998 Elsevier Science B.V. All rights reserved. |
1997 |
| Author(s): Rost, JM; Schulz, K; Domke, M; Kaindl, G |
|
Title:
Resonance parameters of photo doubly excited helium |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 30 (21): 4663-4694 NOV 14 1997 |
| Abstract: Using theoretical results from complex rotation calculations and data from experimental photoionization cross sections, the quantum defects, the widths, the oscillator strengths and the shape parameter of Rydberg series of autoionizing P-1(o) resonances in helium, excited with synchrotron radiation from the ground state, are reviewed and analysed systematically. The relation of these resonance properties to the propensity rules for radiative and non-radiative transitions in two-electron atoms is established. |
| -- |
| Author(s): Malcherek, AW; Rost, JM; Briggs, JS |
|
Title:
Complete photofragmentation of the lithium atom |
| Source: PHYSICAL REVIEW A, 55 (6): R3979-R3982 JUN 1997 |
| Abstract: Differential cross sections describing the correlated motion of three electrons in the nuclear Coulomb field during the complete photofragmentation of a four-body system, the lithium atom, are calculated. Two selection rules are derived and their operation illustrated. Two features, not present in the corresponding three-body case are emphasized, namely that the Wannier configuration, in contrast to the three-body photofragmentation of helium, is allowed. Second, the cross section to access certain spin-momentum configurations of the three particles is zero for an uncorrelated but finite for a correlated final state. |
| -- |
| Author(s): Frank, O; Rost, JM |
|
Title:
Diffraction effects in the photoionization of clusters |
| Source: CHEMICAL PHYSICS LETTERS, 271 (4-6): 367-371 JUN 13 1997 |
| Abstract: In many atomic or molecular clusters an electron cloud exists which is delocalized over a volume of well defined shape. It is shown that the total and partial photoionization cross sections of those clusters oscillate on a scale of energy typically reached by synchrotron radiation. The frequencies of the oscillations are related to geometrical properties of the electron cloud, such as its thickness and the diameter of the cluster. These properties can in principle be extracted from the experimental photo cross section. As specific examples we discuss an alkali-metal cluster (Na-40) and the fullerene C-60. (C) 1997 Published by Elsevier Science B.V. |
| -- |
| Author(s): Grossmann, F; Rost, JM; Schleich, WP |
|
Title:
Spacetime structures in simple quantum systems |
| Source: JOURNAL OF PHYSICS A-MATHEMATICAL AND GENERAL, 30 (9): L277-L283 MAY 7 1997 |
| Abstract: Recently W Kinzel [1995 Phys. BI. 51 1190] has argued that even simple quantum systems can exhibit surprising phenomena. As an example he presented the formation of canals and ridges in the time-dependent probability density of a particle caught in a square well with infinitely high walls. We show how these structures emerge from the wavefunction and present a simple derivation of their location in the spacetime continuum. |
| -- |
| Author(s): Rost, JM; Pattard, T |
|
Title:
Analytical parametrization for the shape of atomic ionization cross sections |
| Source: PHYSICAL REVIEW A, 55 (1): R5-R7 JAN 1997 |
| Abstract: The behavior of the ionization cross section of atoms is known classically in the limits of threshold energy and at high energies. These two limits are used to construct a simple analytical formula for the ionization cross section that depends upon two parameters: the magnitude of the maximum of the cross section and its position in energy. The parametrization has been tested for electron- and positron-impact ionization as well as for proton- and antiproton-impact ionization. It reproduces in all cases the shape of the cross section and offers a unified treatment for ionization by bare projectiles irrespectively of their charge and mass. |
1996 |
| Author(s): Briggs, JS; Kades, E; Manz, J; Rost, JM; Schlier, C; Seiter, A |
|
Title:
A new isotope effect on vibrational states: From hyperspherical modes of H2O to hyperellipsoidal modes of HOT |
| Source: ZEITSCHRIFT FUR PHYSIKALISCHE CHEMIE-INTERNATIONAL JOURNAL OF RESEARCH IN PHYSICAL CHEMISTRY & CHEMICAL PHYSICS, 195: 65-88 Part 1-2 1996 |
| Abstract: Hyperspherical vibrational modes of H2O are distorted into hyperellipsoidal modes of HOT due to symmetry breaking by isotopic substitution. Hyperellipsoidal modes are characterized by vibrational wavefunctions which extend along ellipsoidal-type arcs which cross the dissociative bond coordinates OH, OT perpendicularly. Within the simple two-dimensional Thiele-Wilson model of HOT, we discover a large series of hyperellipsoidal modes which are characterized by v(3) = 9, 10, 11, ... nodes, indicating v(3), quanta in the vibrations along the ellipsoidal-type arcs. The start of this series at v(3) = 9 implies that hyperellipsoidal modes exist only for highly excited energies, greater than or equal to 3.4 eV above the vibrational ground-state. These quantum mechanical results are discovered first empirically, and then explained in terms of illuminating semiclassical and quantum theories. |
| -- |
| Author(s): Schulz, K; Kaindl, G; Domke, M; Bozek, JD; Heimann, PA; Schlachter, AS; Rost, JM |
|
Title:
Observation of new Rydberg series and resonances in doubly excited helium at ultrahigh resolution |
| Source: PHYSICAL REVIEW LETTERS, 77 (15): 3086-3089 OCT 7 1996 |
| Abstract: We report on a striking improvement in spectral resolution in the soft x-ray range to 1.0 meV at 64.1 eV, measured via the mu eV-wide, 2, -l(3) double-excitation resonance of helium. This ultrahigh resolution combined with the high photon flux at undulator beam line 9.0.1 of the Advanced Light Source have allowed observation of new Rydberg series and resonances below the N = 3 threshold of doubly excited He. The obtained resonance parameters (energies, lifetime widths, and Fano-q parameters) are in excellent agreement with the results of state-of-the-art calculations. |
| -- |
| Author(s): Burgers, A; Rost, JM |
|
Title:
Complex expectation values and Lewis structures for resonant states |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 29 (17): 3825-3839 SEP 14 1996 |
| Abstract: The definition of a norm for scattering states is reviewed particularly with respect to the method of complex scaling used to determine resonant states. The meaning of complex expectation values which result from this procedure is illustrated with a simple two-channel model. In a further example autoionizing states of helium are analysed in terms of complex expectation values which are shown to provide a sensitive tool to probe details of complicated dynamics. We also demonstrate how Lewis structures for resonances may be defined through complex expectation values. |
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| Author(s): Frank, O; Rost, JM |
|
Title:
Photoionization of alkali metal clusters |
| Source: ZEITSCHRIFT FUR PHYSIK D-ATOMS MOLECULES AND CLUSTERS, 38 (1): 59-64 AUG 1996 |
| Abstract: The photoionization cross section for spherical alkali metal clusters is predicted to oscillate as a function of the photon wavenumber with a frequency determined by the diameter of the cluster. The oscillations and other principal features of the photo cross section can be worked out analytically using semiclassical techniques. An accurate numerical calculation with different cluster potentials confirms these results qualitatively. Quantitative details depend sensitively on the actual potential. Hence, properties of the true cluster potential can be inferred from the experimental cross section. This might turn out to be useful for improving theoretical cluster potentials. |
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| Author(s): Rost, JM; Wintgen, D |
|
Title:
What happens to two-electron resonances when their energy approaches the break-up threshold? |
| Source: EUROPHYSICS LETTERS, 35 (1): 19-24 JUL 1 1996 |
| Abstract: Using semiclassical S-matrix theory in a simulated electron-hydrogen scattering experiment and converged quantum calculations for the widths of high-lying resonances in H- we show that resonance formation (or the lifetime of resonances) below the fragmentation threshold E = 0 and ionization above threshold have the same energy dependence \E\(1.127) for \E\ --> 0. Hence, series of resonances which have been characterized by approximate quantum numbers will disappear towards threshold. |
| -- |
| Author(s): Rost, JM |
|
Title:
Quantum-classical hybrid approach to helium double photoionization |
| Source: PHYSICAL REVIEW A, 53 (2): R640-R643 FEB 1996 |
| Abstract: The photoionization process is divided into (A) the absorption of the photon by one electron and (B) the correlated motion of the electron pair leading to singly or doubly ionized helium. We relate (A) to the total cross section to be calculated analytically in the quasiclassical reflection approximation as known from molecular problems. For (B) the two-electron wave function is propagated with the semiclassical version of Feynman's path integral to separate single and double ionizing events. A probe for (B) is the ratio between double and single ionization. The results for (A) and (B) and for absolute ionization cross sections obtained by combining (A) and (B) are in good agreement with different experiments that cover together a range of photon energy from the double-ionization threshold to several hundred eV. |
1995 |
| Author(s): Rost, JM |
|
Title:
Analytical total photo cross-section for atoms |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 28 (19): L601-L606 OCT 14 1995 |
| Abstract: A quantum-classical approximation for the total photo cross section of many-electron atoms is derived in a time-dependent formulation. Exact commutator relations for the time-dependent dipole operator and a classical approximation for the propagator are used. Analytical expressions are derived for one- and two-electron atoms. We compare the results for hydrogen with the exact cross section and the high-energy approximation. For helium we show a comparison with experimental data and accurate numerical results. |
| -- |
| Author(s): Bürgers, A; Wintgen, D; Rost, JM |
|
Title:
Highly doubly-excited S-states of the helium atom |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 28 (15): 3163-3183 AUG 14 1995 |
| Abstract: We calculate and analyse S-wave resonances of helium up to an energy of -0.02 au applying the complex rotation technique. Rydberg series converging to the hydrogenic thresholds of the He+ ion are analysed by quantum defect theory. For moderately excited inner electrons the series converging to different thresholds begin to overlap resulting in perturbed Rydberg series. We find that approximate quantum numbers as well as propensity rules governing the decay of the resonances and the perturbation scheme of the Rydberg series prevail. With increasing excitation of the inner electron however, only series whose states tend to extreme interelectronic angles (cos theta) = +/-1 remain regular. |
1994 |
| Author(s): Rost, JM |
|
Title:
Threshold ionizaton of atoms by electron and positron-impact |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 28 (14): 3003-3026 JUL 28 1995 |
| Abstract: In two recent letters results for threshold ionization of hydrogen by electron impact (Rest J M 1994 Phys. Rev. Lett. 72 1998) and positron impact (Rest J M and Heller E J 1994 Phys. Rev. A 49 R4289) were communicated. The results were obtained by calculating the S-matrix semiclassically in Feynman's path integral formalism. This paper gives a more complete account of the theoretical method. Moreover, it is shown how the ionization cross section of atomic targets other than hydrogen can be expressed with the hydrogen cross section through a scaling relation. This demonstrates the universality of the threshold behaviour. |
| -- |
| Author(s): Rost, JM |
|
Title:
The fragmentation threshold in 2-electron atoms - an illustration with classical orbits |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 27 (24): 5923-5931 DEC 28 1994 |
| Abstract: Semiclassical S-matrix theory is applied to electron-hydrogen scattering near the fragmentation threshold E = 0. The relation between resonant scattering (classically chaotic motion for E < 0) and ionization (E > 0) is worked out with emphasis on the qualitative behaviour of the different types of orbit. |
| -- |
| Author(s): Rost, JM |
|
Title:
The molecular-properties of the positronium negative-ion |
| Source: HYPERFINE INTERACTIONS, 89 (1-4): 343-354 1994 |
| Abstract: The dynamics of the positronium negative ion is described in a molecular adiabatic approximation. It is shown, how the classification of the spectrum, propensity rules for radiative and non-radiative transitions and conditions for the occurrence of shape resonances can easily be derived within this approach. Propensity rules for two-photon processes are also derived and used to demonstrate the possibility of an absorption experiment from the ground state to excite a S-1(e) shape resonance, which is unique to Ps(-). |
| -- |
| Author(s): Rost, JM; Heller, EJ |
|
Title:
Ionization of hydrogen by positron-impact near the fragmentation threshold |
| Source: PHYSICAL REVIEW A, 49 (6): R4289-R4292 JUN 1994 |
| Abstract: The semiclassical approximation of Feynman's path integral is used to calculate the S matrix for the positron-impact ionization of hydrogen. The formulation provides a full scattering amplitude, and more importantly does not require knowledge of the asymptotic three-body Coulomb state in the continuum. In the limit of vanishing excess energy, the results confirm Wannier's classical model for fragmentation [Phys. Rev. 90, 817 (1953)]. The experimentally observable ratio of fragmentation versus total ionization (including positronium formation) is predicted. |
| -- |
| Author(s): Thürwächter, R; Rost, JM; Wintgen, D; Briggs, JS |
|
Title:
Lifetimes of 2-electron resonances from an adiabatic approximation |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 27 (12): L275-L280 JUN 28 1994 |
| Abstract: We show that lifetimes of doubly-excited two-electron atoms can be calculated quantitatively from an adiabatic single channel approximation. Surprisingly, this remains true even in cases where the error in the adiabatic resonance position is larger in magnitude than the entire resonance width. |
| -- |
| Author(s): Rost, JM; Heller, EJ |
|
Title:
Semiclassical scattering of charged-particles |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 27 (7): 1387-1395 APR 14 1994 |
| Abstract: The semiclassical scattering amplitude is directly derived from the quantum mechanical S-matrix in the momentum representation. No information about the asymptotic form of the scattering wavefunction is required, which is particularly important for Coulomb problems. We apply the formalism to the scattering of two identical Coulomb particles and prove analytically that the semiclassical result is exact. It does not depend on a particular representation. However, its exactness can be attributed to the existence of a representation in which the quantum and the semiclassical propagator are identical. |
| -- |
| Author(s): Rost, JM |
|
Title:
2-electron escape near-threshold - a classical process |
| Source: PHYSICAL REVIEW LETTERS, 72 (13): 1998-2001 MAR 28 1994 |
| Abstract: For electron impact ionization of hydrogen the S matrix is determined by calculating Feynman's path integral semiclassically. The total ionization probability is in excellent agreement with the experiment. At a critical excess energy of 3.3 eV the differential cross section for the energy sharing between the continuum electrons undergoes a qualitative change which limits the range of the threshold behavior. The Wannier threshold law is confirmed, but only in the limit of vanishing excess energy. |
1993 |
| Author(s): Friedrich, B; Herschbach, DR; Rost, JM; Rubahn, HG; Renger, M; Verbeek, M |
|
Title:
Optical-spectra of spatially oriented molecules - ici in a strong electric-field |
| Source: JOURNAL OF THE CHEMICAL SOCIETY-FARADAY TRANSACTIONS, 89 (10): 1539-1549 MAY 21 1993 |
| Abstract: Strong electric fields can hybridize rotational states of polar molecules and thus create pendular states in which the molecules are confined to librate over a limited angular range about the field direction. In this way, substantial spatial orientation can be attained for the lowest rotational states of many linear, symmetric and asymmetric top molecules. A large fraction of a molecular ensemble can often be condensed into these low rotational states by cooling in a supersonic expansion. Pendular eigenstates differ qualitatively from those of a rotor or an oscillator and can be observed spectroscopically; this provides a means to characterize the extent of the orientation achieved. Here we present high-resolution laser-induced fluorescence spectra of ICl A 3PI1 <-- X1SIGMA0 measured as a function of the electric field strength up to 36 kV cm-1. These spectra are compared with calculated transition frequencies and probabilities between pendulum/pinwheel states. The field-induced mixing of J states changes the transition probabilities markedly and enriches the spectra with many transitions that would be forbidden in the absence of the field. The transition probabilities fluctuate as the angular lobes of the two pendular wavefunctions involved come in and out of phase when the field strength is varied. The observed fluctuations are particularly pronounced in the present case because the dipole moment changes sign between the two electronic states. |
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| Author(s): Rost, JM |
|
Title:
Binding-energies of weakly bound systems from complex dimensional scaling |
| Source: JOURNAL OF PHYSICAL CHEMISTRY, 97 (10): 2461-2463 MAR 11 1993 |
| Abstract: In the limit of infinite spatial dimension D, the particles of a system become perfectly localized at the minimum of an effective potential surface. Perturbation theory in 1/D has been used successfully to predict binding energies and electronic structure in the physical three-dimensional space. For weakly bound systems which are neutral if one particle is removed the large dimensional approach has not been applicable because the effective potential surface exhibits a saddle point rather than a global minimum. We show that the extension to complex dimensional scaling can theoretically handle this situation and gives good practical estimates for the binding energies of three-particle Coulomb systems, calculated here as an application of the new method. |
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| Author(s): Sung, SM; Rost, JM |
|
Title:
Chemical-binding from the infinite dimensional limit |
| Source: JOURNAL OF PHYSICAL CHEMISTRY, 97 (10): 2479-2483 MAR 11 1993 |
| Abstract: The electronic structure of molecules calculated in the limit of infinite spatial dimensions is related to properties in three-dimensional space. In the spirit of the electrostatic theorem, we introduce the concept of antibinding and binding regions to visualize chemical binding. In addition, criteria are developed to determine the ionic or covalent character of the bond. The computational effort requires only the minimization of a multidimensional potential surface at the infinite dimensional limit, yet detailed information can be extracted. Numerical examples are given for the one-electron diatomic molecule. |
1992 |
| Author(s): Rost, JM; Wintgen, D |
|
Title:
Positronium negative-ion - molecule or atom |
| Source: PHYSICAL REVIEW LETTERS, 69 (17): 2499-2502 OCT 26 1992 |
| Abstract: A highly accurate calculation is supplemented by an adiabatic approximation to explore the resonance spectrum of the positronium negative ion (Ps-). Surprisingly, the spectrum can be understood and classified with H-2+ quantum numbers by treating the interelectronic axis of Ps- as an adiabatic parameter. We report and interpret the existence of 1S shape resonances, a phenomenon so far unknown in three-body Coulomb systems. The new results on Ps- combined with previous results for H- suggest the existence of a resonance spectrum and its similarity for all ABA Coulomb systems with charges \Z(A)/Z(B)\ = 1 and masses m(A)/m(B) greater-than-or-equal-to 1. |
| -- |
| Author(s): Rost, JM; Sung, SM; Herschbach, DR; Briggs, JS |
|
Title:
Molecular-orbital description of doubly excited atomic states generalized to arbitrary dimension |
| Source: PHYSICAL REVIEW A, 46 (5): 2410-2419 SEP 1 1992 |
| Abstract: The molecular-orbital description of two-electron atoms [J. M. Feagin and J. S. Briggs, Phys. Rev. A 37, 4599 (1988)], derived from H-2+ by interchanging the roles of electrons and nuclei, is generalized to D dimensions. For H-2+ itself there exist myriad exact interdimensional degeneracies because D --> D + 2 is equivalent to m --> m + 1, augmenting by unity the projection of the electronic angular momentum on the internuclear axis. When the molecular orbitals (MO's) are transcribed to treat two-electron motion, additional constraints limit the exact degeneracies to states in D = 3 and 5, but many approximate degeneracies persist. Since the MO description emphasizes rotational properties of the two-electron atom, the link between dimension and orbital angular momentum is a pervasive feature. We use this link to classify groups of quasidegenerate doubly excited atomic energies and to explain striking similarities among certain pairs of hyperspherical or molecular-orbital two-electron potential curves. |
| -- |
| Author(s): Rost, JM; Griffin, JC; Friedrich, B; Herschbach, DR |
|
Title:
Pendular states and spectra of oriented linear-molecules |
| Source: PHYSICAL REVIEW LETTERS, 68 (9): 1299-1301 MAR 2 1992 |
| Abstract: Recent experiments have demonstrated the feasibility of orienting rotationally cooled polar molecules in an electric field. The anisotropy of the Stark effect allows molecules in low rotational states to be trapped in "pendular states," confined to librate over a limited angular range about the field direction. We present calculations exhibiting the nature of these pendular states for a linear molecule and characteristic features of infrared and microwave spectra which become observable in strong fields. |
1991 |
| Author(s): Rost, JM; Briggs, JS |
|
Title:
Saddle structure of the 3-body Coulomb problem - symmetries of doubly-excited states and propensity rules for transitions |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 24 (20): 4293-4322 OCT 28 1991 |
| Abstract: The separable two-centre Coulomb problem is investigated with emphasis on the saddle structure of its potential. A new quasi-degeneracy between molecular orbitals (MO) is identified. We demonstrate that the results have threefold relevance for the formation and character of symmetric doubly-excited states: (i) it is shown that all features of Herrick's various multiplet classifications can be explained in terms of MO saddle dynamics; (ii) for the first time quantitative results of resonance energies from the adiabatic MO approach are reported and compared to other theoretical and experimental data; (iii) the propensity rules for radiative and non-radiative transitions are summarized and details of their derivation within the framework of motion in the vicinity of the potential saddle are given. |
| -- |
| Author(s): Rost, JM; Briggs, JS |
|
Title:
Mechanisms of electron-impact ionization at threshold |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 24 (16): L393-L396 AUG 28 1991 |
| Abstract: We extend previous analyses of the six-dimensional wavefunction describing threshold ionization and show that the nodal structure near the Wannier saddle can be compactly described by the nodal structure of three-dimensional molecular orbital wavefunctions and their associated quantum numbers. Independent of the Wannier assumption a mechanism is proposed by which only certain states of LS-pi are expected to dominate at threshold. |
| Author(s): Rost, JM; Gersbacher, R; Richter, K; Briggs, JS; Wintgen, D |
|
Title:
The nodal structure of doubly-excited resonant states of helium |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 24 (10): 2455-2466 MAY 28 1991 |
| Abstract: We examine the nodal structure of accurate helium wavefunctions calculated by direct diagonalization of the full six-dimensional problem. It is shown that for fixed interelectronic distance R (or hyperspherical radius R) the symmetric doubly-excited resonant states have well-defined-lambda, mu-nodal structure indicating a near separability in prolate spheroidal coordinates. For fixed-lambda, however, a clear mixing of R, mu-nodes is demonstrated. This corresponds to a breakdown of the adiabatic approximation and can be understood in terms of the classical two-electron motion. |
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| Author(s): Vollweiter, A; Rost, JM; Briggs, JS |
|
Title:
Saddle dynamics of intrashell resonances - propensity rules for radiative transitions |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 24 (6): L155-L159 MAR 28 1991 |
| Abstract: Propensity rules for radiative transitions are given in terms of approximate molecular orbital quantum numbers of doubly excited states. The rules are derived from selection rules for the two-electron centre-of-mass motion linearized around the saddle point of the potential. Two different versions of propensity rules proposed by Sadeghpour and Greene and Gou et al are shown to be special cases of the more general rules presented here. |
| Author(s): Rost, JM; Briggs, JS; Feagin, JM |
|
Title:
Dominant photodetachment channels in H- - comment |
| Source: PHYSICAL REVIEW LETTERS, 66 (12): 1642-1642 MAR 25 1991 |
| Author(s): Rost, JM; Briggs, JS |
|
Title:
Saddle-point resonances in few-body systems |
| Source: CHEMICAL PHYSICS LETTERS, 177 (3): 321-325 FEB 22 1991 |
| Abstract: The generality of resonance formation due to localisation on saddle points of a many-dimensional potential surface governing the motion of few-body systems is demonstrated. Adiabatic potential curves show sequences of avoided crossings. It is shown how diabatic potentials connecting these crossings can be constructed corresponding to preservation of motion on the saddle and leading to a higher degree of fragmentation of the complex. The method is illustrated by calculation of vibrational resonances in type ABA molecules and doubly excited electronic resonances in two-electron atoms. |
| -- |
| Author(s): RICHTER, K; Rost, JM; Thürwächter, R; Briggs, JS; Wintgen, D; Solovev, EA |
|
Title:
New state of binding of antiprotons in atoms |
| Source: PHYSICAL REVIEW LETTERS, 66 (2): 149-152 JAN 14 1991 |
| Abstract: It is suggested that there exists a novel "molecular" state of binding of antiprotons to atoms in which an excited highly polarized electron is located between the nucleus and the antiproton. The lifetime of this state is expected to be decided by the radiative lifetime of the excited electronic state. |
1990 |
| Author(s): Rost, JM; Briggs, JS |
|
Title:
Propensity rules for radiative and nonradiative decay of doubly-excited states |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 23 (14): L339-L346 JUL 28 1990 |
| Abstract: Propensity rules are suggested relating the probability of autoionising and radiative transitions amongst doubly-excited states to their molecular orbital (MO) quantum numbers. A key element of the analysis is the behaviour of the electronic centre of mass on the saddle of the three-body potential. This is an example of a general analysis of stability on a potential ridge given by Fano (1980). |
1989 |
| Author(s): Rost, JM; Briggs, JS |
|
Title:
Diabatic potential curves for symmetric doubly-excited 1Se states - the isoelectronic sequence |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 22 (22): 3587-3602 NOV 28 1989 |
| Abstract: The description of symmetric doubly-excited states of two-electron ions as eigenstates of a diabatic potential for interelectronic motion is extended to the whole two-electron isoelectronic sequence. The authors' calculated energies of infinite series of intrashell states of 1Se symmetry are fitted to a universal double-Rydberg formula for the complete isoelectronic sequence. The underlying dynamics in the formation of such states is shown to be the stabilising influence of rapid interelectronic vibration on the inherently unstable motion of the electronic centre-of-mass around the saddle point of the three-body potential. |
| -- |
| Author(s): Rost, JM; Briggs, JS; Greenland, PT |
|
Title:
New sequences of avoided crossings in the correlation diagram of H2+ and their significance for doubly-excited atomic states |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 22 (13): L353-L359 JUL 14 1989 |
| Abstract: The authors have identified new sequences of avoided crossings between the adiabatic MO of H2+. The diabatic potential connecting them has been obtained and corresponds to the localisation of electron density on the saddle of the two-centre Coulomb potential. This is the direct molecular analogue of the diabatic potential which gives rise to a class of symmetric doubly-excited states of two-electron atoms. In this way a new connection is made between molecular structure and the structure of resonant atomic states. |
1988 |
| Author(s): Rost, JM; Briggs, JS |
|
Title:
Diabatic molecular description of symmetric doubly excited atomic states |
| Source: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 21 (10): L233-L239 MAY 28 1988 |
| Abstract: A new method is presented for the calculation of symmetric doubly excited atomic states. Using a separation in molecular coordinates a diabatic potential for interelectronic vibration is derived with an extremely simple wavefunction containing only one variational parameter. The resulting potential is analytic and is asymptotic to the double-ionisation threshold. Eigenstates of this potential correspond to the (Ns2)1Se sequence of resonant states and their energies are given to high accuracy. The analytic asymptotic Coulomb form of the diabatic potential leads directly to a Rydberg formula for high-lying series of doubly excited states. The validity of such a formula has been suggested repeatedly on the basis of more qualitative or empirical arguments. The method may prove useful in treating other few-dimensional problems in quantum mechanics. |
1987 |
| Author(s): Rost, JM; Briggs, JS |
|
Title:
Adiabatic molecular-energy curves with correct united-atom and separated-atom limits |
| Source: ZEITSCHRIFT FUR PHYSIK D-ATOMS MOLECULES AND CLUSTERS, 5 (4): 339-343 1987 |
| Abstract: The criteria necessary to obtain single-channel adiabatic molecular-energy curves with correct united-atom and separated-atom limits are examined. The simplest LCAO form of molecular orbital allows an explicit analytic demonstration of these criteria to be given. An adiabatic single-channel 1s g molecular energy curve for (d d ) based on two centre functions and satisfying the above limits reproduces the ground-state binding energy of large coupled-channel calculations within 0.7%. |