Atomic Physics 2023

We present a study of alkali-ion (Li, Na, K, Cs) doped Helium droplets at 0 K. We utilize a DFT-based formalism to comprehensively describe the system, yielding results that exhibit good agreement with previously employed Quantum approaches. By significantly expanding the number of particles within our system, we are able to predict the characteristics of the system’s density, as well as any density-dependent observables, for varying particle counts. Notably, these systems exhibit a distinctive density trend near ions, surpassing the freezing density of Helium at 0 K. While the general features (density, number of particles per shell and energy) of these solid-like structures (referred to as "snowballs") are discernible through the DFT approach, certain aspects such as solid ordering remain elusive. To address this, we analyze these systems using the Gaussian Imaginary Time Dependent Hartree method. This approach incorporates the main quantum mechanical effects and enables the system to evolve in a reduced-dimensional space, facilitating the study of ground state properties for relatively large cluster sizes while retaining key quantum features. Our objective involves integrating these two approaches to enhance our DFT representation of large systems and provide a description for the solid-superfluid transition.

Wave propagation through random continuous media remains an important fundamental problem with applications ranging from remote sensing to quantum communication. We refine the methods for accurate numerical simulation and table-top experiments of such media by introducing novel hybrid phase screens. Within this framework, we investigate the effects of disorder on structured light and show how instantaneous spatial singular modes offer improved high-fidelity signal transmission in dynamically evolving media compared to standard encoding bases.

Spontaneous parametric down-conversion (SPDC) is a widely used source for photonic entanglement. Years of focused research have led to a solid understanding of the process, but a cohesive analytical description of the paraxial biphoton state has yet to be achieved. We derive a general expression for the spatio-temporal biphoton state that applies universally across common experimental settings and correctly describes the nonseparability of spatial and spectral modes. We formulate a criterion on how to decrease the coupling of the spatial from the spectral degree of freedom by taking into account the Gouy phase of interacting beams. This work provides new insights into the role of the Gouy phase in SPDC, and also into the preparation of engineered entangled states for multidimensional quantum information processing.

In my work, I investigate strong field physics and the precise control of electron dynamics through the modification of laser fields. The primary focus of my research is on individual silver nanoclusters deposited on surfaces. To accomplish this, our simulations employ the Green Dyadic Method to calculate electric field distributions around nanostructures with arbitrary shapes. These electric field distributions, in turn, facilitate the calculation of photoelectron spectra emitted from these structures, with potential applications in fields like field emission displays, electron microscopy, and advanced sensing technologies.

In order to understand hydrogen-antihydrogen interactions its spectrum has to be known. Until now only the groundstate and few excited states of hydrogen-antihydrogen were calculated. We present excited $\Sigma$ states obtained in the Born-Oppenheimer approximation, which reveal free positronium states within the spectrum.

The generation of phase-locked femtosecond X-ray pulses with controlled time delays would significantly benefit nonlinear spectroscopy. It is known that in a medium inverted by intense SASE pulses from XFEL sources, amplified spontaneous emission exhibits strong temporal coherence and has a duration comparable to the coherence distance of the SASE sub-pulses. Seeding this process adds high transverse coherence. In Ref. [1], it was demonstrated that the generated emission can consist of pairs of pulses with separations and phases that can be extracted from the spectrum. We propose to take a step forward and gain more control over the shape of the produced emission by intelligently modifying the samples. In particular, we suggest using crystals where the Bragg condition is satisfied for both the seed and the produced emission. By generating back-propagating waves, this approach can help synchronize the phases of the sub-pulses and provide more control over their time separations. [1] Zhang, Y., Kroll, T., et al., 2022, Generation of intense phase-stable femtosecond hard X-ray pulse pairs, PNAS, 119(12).

Quantum thermodynamics is gaining more and more popularity as an emerging field, with no lack of theoretical and experimental applications. However, its core foundations and assumptions are still being debated. In particular, the interaction energy between a quantum system and its environment cannot be safely neglected and must be taken into account when discussing the thermodynamic properties of the system. We propose here a strategy to identify how interaction with a thermal bath changes the energy of an open quantum system: based on the existence of an exact time local master equation, we define an effective energy operator and derive fundamental thermodynamic quantities such as work, heat, and entropy production. We derive the first and second laws of thermodynamics and study the connection between violations of the second law and quantum non-Markovianity. Furthermore, we investigate how the presence of initial system-bath correlations may affect the theory and its results.

The ionization of atoms and molecules by strong laser fields has been studied extensively, both theoretically and experimentally. The strong-field approximation (SFA) allows for the analytical solution of the Schrödinger equation and accurately predicts the behavior of ionization processes in intense laser fields. Over the past decade, there has been a growing interest in the study of nondipole effects in these processes. However, such predictions have so far been limited to monochromatic driving laser fields, while experiments often employ quite short pulses. In this paper, we therefore present an extension of the SFA that also allows incorporating the more complicated temporal structure of a few-cycle pulse. By this extension, the prediction of so-called peak shifts is significantly improved, and the ability to control the laser pulse inducing above-threshold ionization is greatly enhanced. The enhanced control over the characteristics of the laser pulse results in more accurate predictions of peak shifts. Our results show better agreement with experimental investigations compared to previous theoretical studies.

Resonance energy transfer between chiral molecules can be used to discriminate between different enantiomers. The transfer rate between chiral molecules consists of nondiscriminatory and discriminatory parts. We show that their ratio is usually larger in the retarded regime or far zone of large separation distances and that the degree of discrimination can be modified when considering a surrounding medium. We highlight the importance of local field effects onto the degree of discrimination, predict for general identical chiral molecules the optimum dielectric medium for discrimination and show that exotic media can even invert the discriminatory effect. When considering a chiral medium the environment itself can actively take part in the discrimination, but the local-field corrections become more involved. We show that the local-field corrections in a chiral medium then lead to a surprising effect in the discrimination.

JAC , the Jena Atomic Calculator, has been developed to support the calculation of atomic structures, processes and cascades. This toolbox aims for providing a general and easy-to-use toolbox for the atomic physics community, including an interface that is equally accessible for code developers as well as scientists working in experiment or theory.

We study binding of $N$ identical heavy fermions by a light atom in two dimensions assuming zero-range attractive heavy-light interactions. By using the mean-field theory valid for large $N$ we show that the $N+1$ cluster is bound when the mass ratio exceeds $1.074N^2$. The mean-field theory, being scale invariant in two dimensions, predicts only the shapes of the clusters leaving their sizes and energies undefined. By taking into account beyond-mean-field effects we find closed-form expressions for these quantities. We also discuss differences between the Thomas-Fermi and Hartree-Fock approaches for treating the heavy fermions.

High harmonic generation (HHG) is a Nobel prize winning phenomenon in 2023 for physics, first observed in noble gases in 1987 [1], wherein a strongly driven electronic system may emit integer multiples of the driving laser field frequency in a nonlinear, nonperturbative up-conversion process. HHG offers an easily controllable source of ultra-short pulses of extreme Ultraviolet (XUV) photons to be used in subsequent attosecond techniques. These pulses, if infused with spin angular momentum via their state of polarisation, may permit the interrogation of chiral properties of matter. However, despite the versatile set of tools used for manipulating the emission, reliable control over the polarisation remains challenging. This is because, in the context of the three-step model [2], the ionised electron is driven away from the originating ion by an elliptically polarised driver – quenching that harmonic emission. One proposed technique to enable circularly polarised harmonic emission to incorporate the so-called “bicircular” field [3] in which two counter-rotating circularly polarised drivers of equal intensity and a 2:1 frequency mixing are combined to form a trefoil-shaped electric field. The critical initial attoseconds, the birth of a chemical reaction, may be observed via ultrafast spectroscopic techniques to monitor the electronic dynamics, excited-state populations, and photo-excitation. We explore the influence of the polarisation state and intensity of driving fields on single atoms, and simple (a)chiral molecules by means of time-dependent density functional theory (TDDFT). This is achieved by probing the electron response in the form of excited state populations, harmonic emission, and ionisation times, in the presence of linearly, elliptically, and bicircularly polarised driving laser fields of varying intensity. Through these investigations, we look through the attosecond window and observe the progression of chemical reactions with greater insight on how they may be manipulated and steered. References [1] A. McPherson, et al., J. Opt. Soc. Am. B 4, 595 (1987). [2] P. Corkum, Phys. Rev. Lett. 71 13, (1994) [3] E. Pisanty et al., Nat. Photon. 13, 569 (2019)

We study the role of resonances in simple models of collisions of electrons and photons with atoms and molecules. To do this we have implemented a one-dimensional multi-channel R-matrix approach based on B-splines to calculate all Siegert states (resonances, bound and virtual states) directly in the complex energy plane. Siegert states are poles of the S-matrix and they influence physical observables greatly. I study few model systems and compare my results with the analytical results, where available. The aim for the future is to use my model to study the xenon giant resonance (motivation is e. g. the article Theoretical characterization of the collective resonance states underlying the xenon giant dipole resonance from Phys. A 91 by Chen in 2015).

We explore the impact of the driving laser's ellipticity and polarization on the low-order harmonic spectrum of benzene and find a strong interference in the 5th harmonic between emission originating from transitions between $\pi$ orbitals and emission from $\sigma$ orbitals. The contribution of the $\pi$ orbitals entirely vanishes due to interference for driving with a laser polarized along a $\sigma_{v}$ mirror axis. However, the $\pi$ orbital's contribution takes over for elliptic polarization while being fundamentally different from the $\sigma$ orbital emission, i.e., having the opposite helicity and a perpendicular major polarization axis. The resulting interference yields a complex dependence of the low-order harmonic spectrum of benzene on the ellipticity and the polarization of the driving field.

The Karlsruhe Tritium Neutrino experiment (KATRIN) tries to measures the mass of the neutrino, by measuring the tritium beta-decay spectrum. Here, the beta-decay spectrum of diatomic tritium containing molecules is used, which can only be evaluated with the help of the molecular final state distribution (FSD). The FSD is, as of now, only available from ab initio calculations. As the KATRIN experiment tries to aim for a relatively high precision, the systematic uncertainties need to be known precisely, i.e. an uncertainty study of the FSD is needed. A new approach to determine the uncertainty was developed, where all parameters used in the FSD calculation were investigated. The uncertainty is determined directly in units of the fitted squared neutrino mass $m_{\nu}^2$ (the important parameter for the KATRIN experiment), by producing a theoretical beta-decay spectrum from a reference FSD, and fitting said beta-decay spectrum with another Test-FSD. Each Test-FSD is generated by varying a single parameter in the FSD calculation, enabling to assign each change in a parameter an uncertainty in $m_{\nu}^2$. The total uncertainty of the FSD is then obtained by combining these singled out uncertainties together.

We consider nonadiabatic above-threshold ionisation by a bichromatic driving beam in the strong-field regime. As the discrete ionisation events are identified by saddle points of the electron's quasi-classical action, we then pose the question of what happens to these saddle points upon a gradual replacement of a beam with its second harmonic. Over this replacement the number of ionisation events per cycle of the fundamental changes from two to four. We want to know which of the four saddle points correspond to the previous two ones, and we are interested in the way they entered the integration contour. For the transition from $\omega$ to $2 \omega$ we find that the way in which the two additional solutions enter the integration contour strongly depends on the electron's asymptotic momentum. The newly emergent saddle points repel the old ones, depending on the sign of the momentum, which hence affects the order in which the new saddle points enter the integration contour. The central object of this behaviour is a complete coalescence of two saddle points, which occurs at zero momentum, and which functions as a branch point: circling around it in parameter space will cause the two saddle points to exchange their labels, thus rendering it impossible to label the saddle points both unambiguously and continuously. That point of coalescence is of special interest to us, as it corresponds to a spectral caustic in the photoelectric response. This spectral caustic involves a total of three saddle points, marking a third-order breakdown of the saddle-point approximation at the coalescence. In addition, the saddle-point coalescence implies a nontrivial topology for the saddle-point set, which we will explore in detail.

Photoionization of atoms and ions is one of the most fundamental atomic processes in light-matter interactions. In the present work, a relativistic single-configuration Dirac-Hartree-Fock method is employed to study angular distribution and linear polarization of the $L\alpha_{1}(3d_{5/2}\rightarrow 2p_{3/2})$, $L\alpha_{2}(3d_{3/2}\rightarrow 2p_{3/2})$, and $L\ell(3s_{1/2}\rightarrow 2p_{3/2})$ lines following innershell $2p_{3/2}$ photoionization of high-$Z$ Ba, Yb, Hg, and Rn atoms by linearly polarized light within the framework of the density matrix theory. It is found that the $L\alpha_{1}$ line is nearly isotropic, whereas the $L\alpha_{2}$ and $L\ell$ lines are weakly anisotropic, which is quite different from the results predicted by Garg $et~al.$ [J. Electron Spectrosc. Relat. Phenom. 248, 147054 (2021)]. In contrast, the presently obtained linear polarization of these lines is not only strong enough to be measurable in experiment, but its dependencies on the nuclear charge $Z$ and ionizing photon energy are also noteworthy to be observed, especially for the weakest $L\ell$ line among them. Such a strong linear polarization and its sizable $Z$-dependence could be expected to be used for exploring electron screening effect in innershell photoionization and also radiative decay dynamics of many-electron atoms.