Dr. Christian S. Kern » People

@article{Kern2023,

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

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

doi = {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{Thomas2022,

title = {A One-Dimensional High-Order Commensurate Phase of Tilted Molecules},

author = {A. Thomas and T. Leoni and O. Siri and C. Becker and M. Unzog and C. S. Kern and P. Puschnig and P. Zeppenfeld},

doi = {10.1039/D2CP00437B},

year  = {2022},

date = {2022-01-01},

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

volume = {24},

pages = {9118-9122},

abstract = {We report on the formation of a high-order commensurate (HOC) structure of 5,14-dihydro-5,7,12,14-tetraazapentacene (DHTAP) molecules on the highly corrugated Cu(110)–(2 × 1)O surface. Scanning tunnelling microscopy shows that the DHTAP molecules form a periodic uniaxial arrangement in which groups of seven molecules are distributed over exactly nine substrate lattice spacings along the [10] direction. DFT-calculations reveal that this peculiar arrangement is associated with different tilting of the seven DHTAP molecules within the quasi one-dimensional HOC unit cell. The orientational degree of freedom thus adds a new parameter, which can efficiently stabilize complex molecular structures on corrugated surfaces.},

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}

}

@article{Presel2022,

title = {Charge and adsorption height dependence of the self-metalation of porphyrins on ultrathin MgO(001) films},

author = {F. Presel and C. S. Kern and T. G. Boné and F. Schwarz and P. Puschnig and M. G. Ramsey and M. Sterrer},

doi = {10.1039/D2CP04688A},

year  = {2022},

date = {2022-01-01},

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

volume = {24},

pages = {28540-28547},

abstract = {We have experimentally determined the adsorption structure, charge state, and metalation state of porphin, the fundamental building block of porphyrins, on ultrathin Ag(001)-supported MgO(001) films by scanning tunneling microscopy and photoemission spectroscopy, supported by calculations based on density functional theory. By tuning the substrate work function to values below and above the critical work function for charging, we succeeded in the preparation of 2H-P monolayers which contain negatively charged and uncharged molecules. Significantly, it is shown that the porphin molecules self-metalate at room temperature, forming the corresponding Mg-porphin, irrespective of their charge state. This is in contrast to self-metalation of tetraphenyl porphyrin (TPP), which occurs on planar MgO(001) only if the molecules are negatively charged. The different reactivity is explained by the reduced molecule-substrate distance of the planar porphin molecule compared to the bulkier TPP. The results of this study shed light on the mechanism of porphyrin self-metalation on oxides and highlight the role of the adsorption geometry on the chemical reactivity.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Egger2020,

title = {Charge-promoted self-metalation of porphyrins on an oxide surface},

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

doi = {10.1002/anie.202015187},

year  = {2021},

date = {2021-01-01},

journal = {Angew. Chem. Int. Ed.},

volume = {60},

pages = {5078-5082},

abstract = {Metalation and self-metalation reactions of porphyrins on oxide surfaces have recently gained interest. The mechanism of porphyrin self-metalation on oxides is, however, far from being understood. Herein, we show by a combination of results obtained with scanning tunneling microscopy, photoemission spectroscopy, and DFT computations, that the self-metalation of 2H-tetraphenylporphyrin on the surface of ultrathin MgO(001) films is promoted by charge transfer. By tuning the work function of the MgO(001)/Ag(001) substrate, we are able to control the charge and the metalation state of the porphyrin molecules on the surface.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Udhardt2017,

title = {Influence of Film and Substrate Structure on Photoelectron Momentum Maps of Coronene Thin Films on Ag(111)},

author = {C. Udhardt and F. Otto and C. S. Kern and D. Lüftner and T. Huempfner and T. Kirchhuebel and F. Sojka and M. Meißner and B. Schröter and R. Forker and P. Puschnig and T. Fritz},

doi = {10.1021/acs.jpcc.7b03500},

year  = {2017},

date = {2017-01-01},

urldate = {2017-01-01},

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

volume = {121},

pages = {12285-12293},

abstract = {Angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) was measured for one-monolayer coronene films deposited on Ag(111). The (kx,ky)-dependent photoelectron momentum maps (PMMs), which were extracted from the ARUPS data by cuts at fixed binding energies, show finely structured patterns for the highest and the second-highest occupied molecular orbitals. While the substructure of the PMM main features is related to the 4 × 4 commensurate film structure, various features with three-fold symmetry imply an additional influence of the substrate. PMM simulations on the basis of both free-standing coronene assemblies and coronene monolayers on the Ag(111) substrate confirm a sizable molecule–molecule interaction because no substructure was observed for PMM simulations using free coronene molecules.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}