Prospects and Limitations of Electronic Structure Imaging by Angle Resolved Photoemission Spectroscopy

The advent of attoscience in the past decade has enabled the investigation of temporal aspects of photoemission. Extending a well-known interferometric technique (RABBITT) from the gas phase to condensed matter enabled us to study energy-dependent photoemission delays. We will show which processes contribute to the observed delays and discuss how these delays relate to the phase of the final state wavepacket. We further demonstrate how RABBITT can be employed to study the local electric field at interfaces and we discuss why precise knowledge of the local field is necessary to study electron dynamics on the attosecond timescale. Locher et al, Rev. Sci. Instr. 85 (2014) 013113 Locher, Castiglioni et al. Optica 2 (2015) 405 Lucchini et al, Phys. Rev. Lett. 115 (2015) 137401

t.b.a.

Michele Puppin, Yunpei Deng, Christopher Nicholson, Johannes Feldl, Hendrik Vita, Claude Monney, Laurenz Rettig, Martin Wolf, and Ralph Ernstorfer There is a long-standing quest for high-repetition rate femtosecond extreme ultraviolet laser sources [1], in particular in view of their application for time-resolved photoelectron spectroscopy. We describe a novel femtosecond XUV source specifically developed for time- and angle-resolved photoelectron spectroscopy (trARPES) of solids. Based on an optical parametric chirped pulse amplifier [2], we achieve a monochromatized photon flux exceeding 10$^{11}$ photons/s at the sample position for a high harmonic with 22 eV photon energy and pulse duration of approximately 20 fs. This light source bridges the technology gap between ultrafast low-repetition rate XUV sources and high-repetition rate UV sources and allows for the mapping of transiently populated conduction bands throughout the entire Brillouin zone. TrARPES additionally provides access to ultrafast excited state electron dynamics as we demonstrate for the semiconducting transition metal dichalcogenide WSe$_2$. [1] F. Lindner et al., Phys. Rev. A 68 013814 (2003); C. Heyl et al., J. Phys. B: At. Mol. Opt. Phys. 45, 074020 (2012); M. Krebs et al., Nature Photon. 7, 555 (2013); C.-T. Chiang et al., New J. Phys. 17, 013035 (2015), H. Wang et al., Nature Comm. 6:7459 (2015). [2] M. Puppin et al., Opt. Exp. 23 1491 (2015).

Photoelectron momentum mapping with high energy resolution allows imaging of molecular orbitals with vibronic resolution. We demonstrate that the intensity patterns of photoelectrons derived for the vibronic sidebands of molecular states show characteristic changes due to the distortion of the molecular frame in the vibronically excited state. By a comparison to the simulated patterns derived from calculations, an assignment of specific vibronic modes that preferentially couple to the electronic excitation is possible. In the example of the highest occupied molecular orbital of coronene two inplane modes with an energy of 195\,meV and 196\,meV, respectively, agree with the experimental momentum patterns. Symmetry arguments prefer the latter mode, which is A$_{\rm g}$-symmetric. Orbital imaging by photoelectron momentum mapping with vibronic resolution thus provides unique information for the analysis of the coupling between electronic and vibronic excitation, and allows fascinating insight into the properties of molecular materials.

Metal-organic interfaces play a crucial role for the electronic functionality and overall performance of organic devices. Such interfaces furthermore provide an interesting playground to study fundamental physical effects such as interaction mechanisms and their influence on the electronic structure. In case of particular adsorbate/substrate combinations the lowest unoccupied molecular orbital (LUMO) is (partially) occupied due to hybridization with and charge transfer from the metal substrate [1]. Using high-resolution photoelectron spectroscopy (PES), for some systems (amongst them phthalocyanines (Pc) and 1,4,5,8-naphtalene tetracarboxylic dianhydride on metall surfaces) a peculiar, very narrow feature can be observed at the Fermi energy, whose origin is believed to be a consequence of strong electronic correlations between molecular states and the Bloch states of the substrate [2]. At low temperatures (T < 20K), the line-width of this feature amounts to only ~ 10 meV, representing an unusually small energy scale for electronic excitations in these systems. Moreover, the signal displays strong temperature dependence and is directly connected to the binding energy of the LUMO. For a different combination, namely FePc on Ag(110), the former LUMO also hybridizes and is pulled below the Fermi level [3]. If measured with high-resolution PES one can recognize an energetic splitting of this former degenerate orbital with a separation of roughly 200 meV. Angle-resolved PES results show that this behaviour is explained by a breaking of the molecular four-fold symmetry due to adsorption on the two-fold symmetric substrate. REFERENCES 1. Y. Zou et al., Surf. Sci 600, 1240 (2006) and J. Ziroff et al, Phys Rev. Lett. 104, 233004 (2010). 2. A. Mugarza, Nature Comm. 2, 490 (2011) and J. Ziroff et al., Phys. Rev. B 85, 161404(R) (2012). 3. V. Feyer et al, Surf. Sci. 621 64 (2014).

Heteromolecular monolayer films consisting of two different types of molecules are promising candidates for tailoring the geometric and electronic properties at metal-organic hybrid interfaces. So far, this approach has only been demonstrated on noble metal surfaces where the molecules show a weak chemical molecule-substrate interaction [B. Stadtmüller, Nat. Commun. 5, 3685 (2014)]. In this study, we extend this concept to ferromagnetic surfaces by investigating the formation of heteromolecular CuPc-C60 structures on the Co(1000) surface. On such transition metal surfaces, the strong molecule-substrate interaction usually prevents the formation of long-range ordered molecular structures. We have studied the electronic and structural properties of the heteromolecular CuPc-C60/Co(1000) system using a k-space photoemission microscope. Analyzing the angular dependent photoemission pattern allows us to assign spectroscopic features to molecular orbitals and to determine the orientation of corresponding molecules on the surface. A comparison of these results to the CuPc and C60 homomolecular films on Co(1000) reveals that the formation of ordered heteromolecular films is mainly caused by a site-specific interaction of C60 with Co.

P. Kliuiev, T. Latychevskaia, H.-W. Fink, J. Osterwalder, M. Hengsberger, and L. Castiglioni We present a technique for the complete reconstruction of molecular electronic wave functions including the phase, from angular resolved photoelectron spectroscopy (ARPES) data. ARPES of oriented molecules on single-crystalline metal substrates was shown to provide rich information on the molecular orbital structure [1-3]. If the photoelectron final state can be treated as a plane wave, then the ARPES intensity is proportional to the squared modulus of the Fourier transform of the initial state wave function [1]. The phase of the complex-valued photoelectron distribution in the detector plane is lost in the measurement, but can be found by adapting iterative phase retrieval methods known in optics [4,5], provided the intensity in the detector plane is measured at so-called oversampling condition and some a priori knowledge about the object is known [4,5]. In this work, we draw an analogy between the molecular orbital imaging via ARPES and optical coherent diffractive imaging of microstructures and discuss the main problems of related phase retrieval. ?We present molecular orbital reconstructions from simulated and experimental ARPES data and address the limitations of the plane wave approximation for the final state [6]. [2] Lueftner, D. et al. Imaging the wave functions of adsorbed molecules. PNAS 2014, 111, 2, 605-610. [3] Wiessner, M. et al. Complete determination of molecular orbitals by measurement of phase symmetry and electron density. Nat. Com. 2014, 5, 4156. [4] Fienup, J. R. Reconstruction of an Object from the Modulus of Its Fourier Transform. Optics letters 1978, 3, 27–29. [5] Fienup, J. R. Phase Retrieval Algorithms: A Comparison. Applied optics 1982, 21, 2758–69. [6] Bradshaw A. et al. Molecular orbital tomography for adsorbed molecules: is a correct description of the final state really important? New Journal of Physics 2015, 17, 013033.

Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique to investigate the electronic structure of organic films at the interface to metallic surfaces. However, the experimental data is sometimes difficult to interpret owing to photoemission selection rules which depend on the chosen experimental geometry such polarization vector, angle of incidence and photon energy of the incoming light. Therefore, ARPES simulations based on ab-initio calculations within the framework of density functional theory (DFT) are of great interest. A particularly simple and at the same time powerful approach is to approximate the final state by a plane wave. This renders the ARPES data to be proportional to the Fourier transform of the initial state leading to a technique sometimes termed orbital tomography [1]. Thereby, detailed information on the spatial distribution of the initial state orbital can be obtained. However, the validity of the plane wave approximation has been questioned, in particular, in the limit of small molecules [2]. Therefore in this contribution, I investigate the electronic structure of a benzene ring adsorbed on a Pd(110) surface. After exploring the energetically most favorable adsorption sites by making use of state-of-the-art van-der-Waals corrected DFT calculations, I perform ARPES simulations for several substrate-adsorbate-geometries. When compared to experimental data, the adsorption-site sensitivity which is found in theoretical results allows us to draw conclusions on the optimum adsorption site and orientation of benzene on Pd(110). We discuss the applicability of the plane wave approximation for the final state for such rather small molecules and present an approach which allows us to go beyond the plane wave approximation. [1] P. Puschnig et al, Science 326, 702 (2009). [2] A. M. Bradshaw, and D. P. Woodruff, New J. Phys. 17, 013033 (2015).

Authors: P. Krüger and S. Moser Titanium oxide is used for various chemical and opto-electronic applications. Detailed understanding of the electronic structure is important for designing new titania based nanostructures [1] and functional materials. Angle resolved resonant photoemission (RESPES) can provide local information about the wave function distribution of selected valence states [2]. Here we analyze RESPES data of weakly n-doped anatase TiO2 in order to shed light on the orbital character of the carrier states. The constant initial state spectrum of the Fermi surface resonance is very different from the absorption spectrum, which can qualitatively be understood by projecting the absorption spectrum onto the orbital (xy) character of the conduction band states [3]. The data is further analyzed using an atomic multiplet approach and the results are rationalized in terms of a dynamical change of the d-shell occupation on the core-hole sites. [1] Xiaohui Zhu, Adam Hitchcock, Carla Bittencourt, Polona Umek and Peter Krüger, J. Phys. Chem. C 119, 24192 (2015). [2] P. Krüger, J. Jupille, S. Bourgeois, B. Domenichini, A. Verdini, L. Floreano and A. Morgante, Phys. Rev. Lett. 108, 126803 (2012) [3] P. Krüger, Phys. Rev. B 81, 125121 (2010).

The frontier electronic orbitals of molecules are the prime determinants of the respective compounds’ chemical, electronic, and optical properties. Therefore an experimental determination of orbitals is desirable. However, only the electron densities and energy levels are directly observable, and regardless whether orbitals are observed in real space with scanning probe experiments, or in reciprocal space by photoemission, the phase information of the orbital is lost in the experiment. Here we show, that two dimensional molecular orbital images as well as the absent phase information can be retrieved by applying an iterative procedure, which takes measured photoemission momentum maps as an input, requires no a priori knowledge of the orbitals. The method is based on the assumption of the final state of the photoemission process as a plane wave as well as the spatial confinement of the orbital [1]. Information on the third dimension of the orbital may be obtained using photoemission initial state scans as a function of photon energy. Here, the experimental data exhibits an additional modulations, beyond the plane wave final state, which may be attributed to scattering effects. Nevertheless, as these effects are comparably small, it is possible to obtain three-dimensional images of individual molecular orbitals [2]. The method is demonstrated for the LUMO and HOMO of 3,4,9,10-perylene tetracarboxylic dianhydride. [1] D. Lüftner et al., Proc. Natl. Acad. Sci. (USA) 111, 605 (2014) [2] S. Wei{\ss} et al., Nat. Commun. 6, 8287 (2015)

We model the angle-resolved photoemission spectra (ARPES) of graphene on hexagonal boron nitride (hBN) and show their characteristic features arising due to the formation of miniband structure for graphene electrons in the periodic moir\'{e} pattern. We show that detailed analysis of these features can be used to pin down the microscopic mechanism of the interaction between graphene and hBN. We also show how the presence of a moir\'{e}-periodic strain in graphene or scattering of photoemitted electrons off hBN can be distinguished from the miniband formation.

During the last decades, hybrid interfaces between organic and inorganic materials have attracted great interest not only due to their high relevance for future electronic and spintronic applications. They are also model systems which provide an ideal playground to study interactions across such interfaces and how these interactions determine experimental observables such as the vertical bonding distance or the energy level alignment. In this presentation, we will employ momentum microscopy to study the momentum space signature of molecular orbitals at metal-organic interfaces. This approach allows us to assign molecular photoemission emission features to the emitting molecular orbitals and to gain insight into geometric properties of molecular films. In particular, we will focus on different ways to tailor the electronic (and thus also the geometric and spin-specific) properties of hybrid interfaces either “from the top” by Cs doping of the adsorbate layer or “from the bottom” by including a Pb spacer layer. In addition, we will present time-resolved momentum microcopy results obtained for interfaces between prototypical molecules and a Pb spacer layer on Ag(111). These results will show the potential of time-resolved momentum microscopy to directly obtain information about the momentum resolved lifetime of excited electrons and to trace their relaxation pathways in momentum space.

t.b.a.

In a collaboration of University of Mainz and MPI Halle we combined ToF detection of photoelectrons with a momentum microscope. The momentum microscope offers the unique possibility to measure both, momentum and real space images of the photoemitted electrons with highest efficiency. In combination with a hemispherical analyzer k-microscopy has previously set a benchmark in k-resolution. Here we report on the ToF approach, where the complete momentum space is measured in parallel. Recent improvement in energy and momentum resolution ($\Delta$E=20 meV, $\Delta$k=0.01 1/A) enabled us to resolve the spin splitting of the unoccupied part of the Shockley surface state in cesiated Au(111) by 2PPE. Sub-monolayer coverage of Cs shifts the work function until we reach the threshold for 1PPE. The small energy distribution of the emitted electrons at that point minimizes the chromatic aberrations in the real-space PEEM mode, giving rise to very high spatial resolution. In this way we were able to resolve smallest details on the surface with 300 nm resolution, even without employing a k-aperture which will improve the base resolution to <50 nm. Beside the Cs-Au(111) results we will show results from our progress towards measuring topological systems like graphene on Ir(111) and Bi$_2$Se$_3$.

In this work we present a comprehensive characterization of the geometrical and electronic structure of self-assembled nickel-tetraphenylporphyrin (Ni-TPP) films on Cu(100). The adsorption of the Ni-TPP was studied by a multi-technique approach combining scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED) and Angle-Resolved Photoemission Spectroscopy (ARPES) complemented by density functional theory (DFT). Characterization at the nanoscale by STM reveals the presence of two distinct domains (rotated by an angle of 16\textdegree. The angle between the molecular axis and [110] crystal direction is of \pm 18\textdegree. Simulations based on the observed structure are in good agreement with the measured LEED pattern. In the next step ARPES was employed to investigate the electronic structure at the molecule/metal interface of Ni-TPP/Cu(100) system. The VB spectra of Ni-TPP/Cu(100) system show two features originating from the ionization of low energy MOs. Those features have been analyzed by molecular tomography method and compared to DFT calculations. In the simulated ARPES image the symmetry of the system and the angle between the two molecules which belong to different domains were taken into account. The image shows is in good agreement between the experimentally measured pattern and the simulated one. The presence of LUMO in the photoemisson spectrum supports the charge transfer scenario between molecules and the metal substrate. In conclusion, the adsorption and growth behavior of Ni-TPP molecule on the Cu(100) surface has been studied using different spectroscopic techniques, providing information regarding their geometric and electronic structure.