Dr. Christian S. Kern » People

@article{Kern2026,

title = {Circular dichroism in the photoelectron angular distribution of achiral molecules},

author = {C. S. Kern and X Yang and G. Zamborlini and S. Mearini and M. Jugovac and V. Feyer and U. De Giovannini and A. Rubio and S. Soubatch and M. G. Ramsey F. S. Tautz and P. Puschnig},

url = {https://journals.aps.org/prresearch/abstract/10.1103/6bkb-4rm3},

doi = {10.1103/6bkb-4rm3},

year  = {2026},

date = {2026-06-11},

journal = {Phys. Rev. Research},

volume = {8},

issue = { 023275},

abstract = {Circular dichroism in the angular distribution (CDAD) is the effect that the angular intensity distribution of photoemitted electrons depends on the handedness of the incident circularly polarized light. The origin of CDAD can be manifold, including intrinsic properties of the system under study, such as chirality, spin-orbit interaction, or quantum-geometrical properties, but CDAD can also originate from final-state effects influenced by the experimental geometry. For example, CDAD has been reported for achiral organic molecules at the interface to metallic substrates. For this latter case, we investigate two prototypical 𝜋

-conjugated molecules, namely, tetracene and pentacene, whose frontier orbitals have a similar shape but exhibit distinctly different symmetries. By comparing experimental CDAD momentum maps with simulations within time-dependent density functional theory, we show how the final state of the photoelectron must be regarded as the source of the CDAD in such otherwise achiral and quantum-geometrically trivial systems. We gain additional insight into the mechanism by employing a simple scattering model for the final state, which allows us to decompose the CDAD signal into partial wave contributions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Theilen2025,

title = {Observing the spatial and temporal evolution of exciton wave functions},

author = {M. Theilen and S. Kaidisch and M. Stettner and S. Zajusch and E. Fackelman and A. Adamkiewicz and R. Wallauer and A. Windischbacher and C. S. Kern and M. G. Ramsey and F. C. Bocquet and S. Soubatch and F. S. Tautz and U. Höfer and P. Puschnig},

url = {https://arxiv.org/abs/2511.23001},

doi = {10.48550/arXiv.2511.23001},

year  = {2025},

date = {2025-11-28},

urldate = {2025-11-28},

journal = { arXiv:2511.23001  [cond-mat.mtrl-sci]},

abstract = {Excitons, the correlated electron-hole pairs governing optical and transport properties in organic semiconductors, have long resisted direct experimental access to their full quantum-mechanical wave functions. Here, we use femtosecond time-resolved photoemission orbital tomography (trPOT), combining high-harmonic probe pulses with time- and momentum-resolved photoelectron spectroscopy, to directly image the momentum-space distribution and ultrafast dynamics of excitons in  -sexithiophene thin films. We introduce a quantitative model that enables reconstruction of the exciton wave function in real space, including both its spatial extent and its internal phase structure. The reconstructed wave function reveals coherent delocalization across approximately three molecular units and exhibits a characteristic phase modulation, consistent with ab initio calculations within the framework of many-body perturbation theory. Time-resolved measurements further show a  % contraction of the exciton radius within 400 fs, providing direct evidence of self-trapping driven by exciton-phonon coupling. These results establish trPOT as a general and experimentally accessible approach for resolving exciton wave functions -- with spatial, phase, and temporal sensitivity -- in a broad class of molecular and low-dimensional materials. },

keywords = {},

pubstate = {forthcoming},

tppubtype = {article}

}

@article{Kaidisch2025_arxiv,

title = {Photoemission tomography of excitons in 2D systems: momentum-space signatures of correlated electron-hole wave functions},

author = {S. Kaidisch and A. Kleiner and S. Refaely-Abramson and P. Puschnig and C. S. Kern},

url = {https://arxiv.org/abs/2511.14956},

doi = {10.48550/arXiv.2511.14956},

year  = {2025},

date = {2025-11-18},

urldate = {2025-11-18},

journal = { arXiv:2511.14956 [cond-mat.mtrl-sci]},

abstract = {The momentum-space signatures of excitons can be experimentally accessed through time-resolved (pump-probe) photoelectron spectroscopy. In this work, we develop a computational framework for exciton photoemission orbital tomography (exPOT) in periodic systems, enabling the simulation and interpretation of experimental observables within many-body perturbation theory. By connecting the  +Bethe-Salpeter Equation (BSE) approach to photoemission tomography, our formalism captures exciton photoemission in periodic systems, explicitly incorporating photoemission matrix element effects induced by the probe pulse. The correlated nature of electrons and holes introduces distinct consequences for excitonic photoemission, including a dependence on pump pulse polarization. Using the prototypical two-dimensional material hexagonal boron nitride, we demonstrate these effects and show how our framework extends to excitons with finite center-of-mass momentum, making it well-suited to studying momentum-dark excitons. This provides valuable insights into the microscopic nature of excitonic phenomena in quantum materials. },

keywords = {},

pubstate = {forthcoming},

tppubtype = {article}

}

@article{Kern2025,

title = {Circular dichroism in the photoelectron angular distribution of achiral molecules},

author = {C. S. Kern and X. Yang and G. Zamborlini and S. Mearini and M. Jugovac and V. Feyer and U. De Giovannini and A. Rubio and S. Soubatch and M. G. Ramsey and F. S. Tautz and P. Puschnig},

url = {https://arxiv.org/abs/2507.12113},

doi = {10.48550/arXiv.2507.12113},

year  = {2025},

date = {2025-07-16},

urldate = {2025-07-16},

journal = {arXiv:2507.12113 [cond-mat.mtrl-sci]},

keywords = {},

pubstate = {forthcoming},

tppubtype = {article}

}

@article{Hurdax2025,

title = {Integer Charge Transfer Model–PTCDA on MgO(001)/Ag(001) Probing the Transition from Single to Double Integer Charge Transfer},

author = {P. Hurdax and M. Hollerer and C. S. Kern and P. Puschnig and M. Sterrer and M. G. Ramsey},

url = {https://pubs.acs.org/doi/10.1021/acs.jpcc.4c08104},

doi = {10.1021/acs.jpcc.4c08104},

issn = {1932-7455},

year  = {2025},

date = {2025-01-08},

urldate = {2025-01-08},

journal = {J. Phys. Chem. C},

volume = {129},

issue = {2},

pages = {1553--1561},

publisher = {American Chemical Society (ACS)},

abstract = {For weakly interacting adsorbate/substrate systems, the integer charge transfer (ICT) model describes how charge transfer across interfaces depends on the substrate work function. In particular, work function regimes where no charge transfer occurs (vacuum level alignment) can be distinguished from regions where integer charge transfer by electron tunneling from substrate to adsorbate or vice versa takes place (Fermi level pinning). While the formation of singly integer charged molecular anions and cations of organic semiconductors on various substrates has been well described by this model, the double integer charging regime has so far remained unexplored and experimentally elusive. Here, we extend the integer charge transfer model to the transition from single to double integer charging. This was made possible by combining a molecular adsorbate with high electron affinity (Perylenetetracarboxylic-dianhydride (PTCDA)) with a substrate with tunable work function (ultrathin MgO(001) films on Ag(001)). Our results, obtained with scanning tunneling microscopy (STM), photoemission spectroscopy (PES), work function measurements and density function theory (DFT) calculations, show that after completing the single negative charging of all molecules in a PTCDA monolayer in the first Fermi level pinning regime, the system transitions to a vacuum level alignment regime for singly charged molecules when the substrate work function is reduced, and finally enters the second Fermi level pinning regime at very low substrate work function, in which the molecules become doubly negatively charged.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bennecke2024,

title = {Disentangling the multiorbital contributions of excitons by photoemission exciton tomography},

author = {W. Bennecke and A. Windischbacher and D. Schmitt and J. P. Bange and R. Hemm and C. S. Kern and G. D’Avino and X. Blase and D. Steil and S. Steil and M. Aeschlimann and B. Stadtmüller and M. Reutzel and P. Puschnig and G. S. M. Jansen and S. Mathias},

url = {https://www.nature.com/articles/s41467-024-45973-x},

doi = {10.1038/s41467-024-45973-x},

year  = {2024},

date = {2024-02-28},

urldate = {2024-02-28},

journal = {Nature Communications},

volume = {15},

number = {1804},

pages = {10},

abstract = {Excitons are realizations of a correlated many-particle wave function, specifi-cally consisting of electrons and holes in an entangled state. Excitons occurwidely in semiconductors and are dominant excitations in semiconductingorganic and low-dimensional quantum materials. To efficiently harness thestrong optical response and high tuneability of excitons in optoelectronics andin energy-transformation processes,access to the full wavefunction of theentangled state is critical, but has so far not been feasible. Here, we show howtime-resolved photoemission momentum microscopy can be used to gainaccess to the entangled wavefunction and to unravel the exciton’s multiorbitalelectron and hole contributions. For the prototypical organic semiconductorbuckminsterfullerene (C60), we exemplify the capabilities of exciton tomo-graphy and achieve unprecedented access to key properties of the entangledexciton state including localization, charge-transfer character, and ultrafastexciton formation and relaxation dynamics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kern2023,

title = {Photoemission orbital tomography for excitons in organic molecules},

author = {C. S. Kern and A. Windischbacher and P. Puschnig},

doi = {https://doi.org/10.1103/PhysRevB.108.085132},

year  = {2023},

date = {2023-08-22},

urldate = {2023-08-22},

journal = {Phys. Rev. B},

volume = {108},

pages = {085132},

abstract = {Driven by recent developments in time-resolved photoemission spectroscopy, we extend the successful method of photoemission orbital tomography (POT) to excitons. Our theory retains the intuitive orbital picture of POT, while respecting both the entangled character of the exciton wave function and the energy conservation in the photoemission process. Analyzing results from three organic molecules, we classify generic exciton structures and give a simple interpretation in terms of natural transition orbitals. We validate our findings by directly simulating pump-probe experiments with time-dependent density functional theory.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kern2023b,

title = {Simple extension of the plane-wave final state in photoemission: Bringing understanding to the photon-energy dependence of two-dimensional materials},

author = {C. S. Kern and A. Haags and L.Egger and X. Yang and H. Kirschner and S. Wolff and T. Seyller and A. Gottwald and M. Richter and U. De Giovannini and A. Rubio and M. G. Ramsey and F. C. Bocquet and Soubatch and F. S. Tautz and P. Puschnig and S. Moser},

url = {https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.5.033075},

doi = {10.1103/PhysRevResearch.5.033075},

year  = {2023},

date = {2023-08-03},

journal = {Phys. Rev. Research},

volume = {5},

issue = {033075},

abstract = {Angle-resolved photoemission spectroscopy (ARPES) is a method that measures orbital and band structure contrast through the momentum distribution of photoelectrons. Its simplest interpretation is obtained in the plane-wave approximation, according to which photoelectrons propagate freely to the detector. The photoelectron momentum distribution is then essentially given by the Fourier transform of the real-space orbital. While the plane-wave approximation is remarkably successful in describing the momentum distributions of aromatic compounds, it generally fails to capture kinetic-energy-dependent final-state interference and dichroism effects. Focusing our present study on quasi-freestanding monolayer graphene as the archetypical two-dimensional (2D) material, we observe an exemplary 𝐸kin

-dependent modulation of, and a redistribution of spectral weight within, its characteristic horseshoe signature around the

‾‾‾

K

and

‾‾‾

K

′

points: both effects indeed cannot be rationalized by the plane-wave final state. Our data are, however, in remarkable agreement with ab initio time-dependent density functional simulations of a freestanding graphene layer and can be explained by a simple extension of the plane-wave final state, permitting the two dipole-allowed partial waves emitted from the C 2⁢𝑝𝑧

orbitals to scatter in the potential of their immediate surroundings. Exploiting the absolute photon flux calibration of the Metrology Light Source, this scattered-wave approximation allows us to extract 𝐸kin

-dependent amplitudes and phases of both partial waves directly from photoemission data. The scattered-wave approximation thus represents a powerful yet intuitive refinement of the plane-wave final state in photoemission of 2D materials and beyond.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hurdax2022,

title = {Large Distortion of Fused Aromatics on Dielectric Interlayers Quantified by Photoemission Orbital Tomography},

author = {P. Hurdax and C. S. Kern and T. G. Boné and A. Haags and M. Hollerer and L. Egger and X. Yang and H. Kirschner and A. Gottwald and M. Richter and F. C. Bocquet and S. Soubatch and G. Koller and F. S. Tautz and M. Sterrer and P. Puschnig and M. G. Ramsey},

doi = {10.1021/acsnano.2c08631},

year  = {2022},

date = {2022-01-01},

journal = {ACS Nano},

volume = {16},

pages = {17435-17443},

abstract = {Polycyclic aromatic compounds with fused benzene rings offer an extraordinary versatility as next-generation organic semiconducting materials for nanoelectronics and optoelectronics due to their tunable characteristics, including charge-carrier mobility and optical absorption. Nonplanarity can be an additional parameter to customize their electronic and optical properties without changing the aromatic core. In this work, we report a combined experimental and theoretical study in which we directly observe large, geometry-induced modifications in the frontier orbitals of a prototypical dye molecule when adsorbed on an atomically thin dielectric interlayer on a metallic substrate. Experimentally, we employ angle-resolved photoemission experiments, interpreted in the framework of the photoemission orbital tomography technique. We demonstrate its sensitivity to detect geometrical bends in adsorbed molecules and highlight the role of the photon energy used in experiment for detecting such geometrical distortions. Theoretically, we conduct density functional calculations to determine the geometric and electronic structure of the adsorbed molecule and simulate the photoemission angular distribution patterns. While we found an overall good agreement between experimental and theoretical data, our results also unveil limitations in current van der Waals corrected density functional approaches for such organic/dielectric interfaces. Hence, photoemission orbital tomography provides a vital experimental benchmark for such systems. By comparison with the state of the same molecule on a metallic substrate, we also offer an explanation why the adsorption on the dielectric induces such large bends in the molecule.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}