@article{TetenoireEhlertJuaristietal.2022, author = {Tetenoire, Auguste and Ehlert, Christopher and Juaristi, Joseba I{\~n}aki and Saalfrank, Peter and Alducin, Maite}, title = {Why ultrafast photoinduced CO desorption dominates over oxidation on Ru(0001)}, series = {The journal of physical chemistry letters}, volume = {13}, journal = {The journal of physical chemistry letters}, number = {36}, publisher = {American Chemical Society}, address = {Washington, DC}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.2c02327}, pages = {8516 -- 8521}, year = {2022}, abstract = {CO oxidation on Ru(0001) is a long-standing example of a reaction that, being thermally forbidden in ultrahigh vacuum, can be activated by femtosecond laser pulses. In spite of its relevance, the precise dynamics of the photoinduced oxidation process as well as the reasons behind the dominant role of the competing CO photodesorption remain unclear. Here we use ab initio molecular dynamics with electronic friction that account for the highly excited and nonequilibrated system created by the laser to investigate both reactions. Our simulations successfully reproduce the main experimental findings: the existence of photoinduced oxidation and desorption, the large desorption to oxidation branching ratio, and the changes in the O K-edge X-ray absorption spectra attributed to the initial stage of the oxidation process. Now, we are able to monitor in detail the ultrafast CO desorption and CO oxidation occurring in the highly excited system and to disentangle what causes the unexpected inertness to the otherwise energetically favored oxidation.}, language = {en} } @misc{MayerLeverPicconietal.2022, author = {Mayer, Dennis and Lever, Fabiano and Picconi, David and Metje, Jan and Ališauskas, Skirmantas and Calegari, Francesca and D{\"u}sterer, Stefan and Ehlert, Christopher and Feifel, Raimund and Niebuhr, Mario and Manschwetus, Bastian and Kuhlmann, Marion and Mazza, Tommaso and Robinson, Matthew Scott and Squibb, Richard James and Trabattoni, Andrea and Wallner, M{\aa}ns and Saalfrank, Peter and Wolf, Thomas J. A. and G{\"u}hr, Markus}, title = {Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy}, series = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {1301}, issn = {1866-8372}, doi = {10.25932/publishup-57744}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-577442}, pages = {9}, year = {2022}, abstract = {The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220-250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.}, language = {en} } @article{MayerLeverPicconietal.2022, author = {Mayer, Dennis and Lever, Fabiano and Picconi, David and Metje, Jan and Ališauskas, Skirmantas and Calegari, Francesca and D{\"u}sterer, Stefan and Ehlert, Christopher and Feifel, Raimund and Niebuhr, Mario and Manschwetus, Bastian and Kuhlmann, Marion and Mazza, Tommaso and Robinson, Matthew Scott and Squibb, Richard James and Trabattoni, Andrea and Wallner, M{\aa}ns and Saalfrank, Peter and Wolf, Thomas J. A. and G{\"u}hr, Markus}, title = {Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy}, series = {Nature Communications}, volume = {13}, journal = {Nature Communications}, publisher = {Springer Nature}, address = {Berlin}, issn = {2041-1723}, doi = {10.1038/s41467-021-27908-y}, pages = {9}, year = {2022}, abstract = {The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220-250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.}, language = {en} } @article{EhlertKlamroth2020, author = {Ehlert, Christopher and Klamroth, Tillmann}, title = {PSIXAS: A Psi4 plugin for efficient simulations of X-ray absorption spectra based on the transition-potential and Delta-Kohn-Sham method}, series = {Journal of computational chemistry : organic, inorganic, physical, biological}, volume = {41}, journal = {Journal of computational chemistry : organic, inorganic, physical, biological}, number = {19}, publisher = {Wiley}, address = {Hoboken}, issn = {0192-8651}, doi = {10.1002/jcc.26219}, pages = {1781 -- 1789}, year = {2020}, abstract = {Near edge X-ray absorption fine structure (NEXAFS) spectra and their pump-probe extension (PP-NEXAFS) offer insights into valence- and core-excited states. We present PSIXAS, a recent implementation for simulating NEXAFS and PP-NEXAFS spectra by means of the transition-potential and the Delta-Kohn-Sham method. The approach is implemented in form of a software plugin for the Psi4 code, which provides access to a wide selection of basis sets as well as density functionals. We briefly outline the theoretical foundation and the key aspects of the plugin. Then, we use the plugin to simulate PP-NEXAFS spectra of thymine, a system already investigated by others and us. It is found that larger, extended basis sets are needed to obtain more accurate absolute resonance positions. We further demonstrate that, in contrast to ordinary NEXAFS simulations, where the choice of the density functional plays a minor role for the shape of the spectrum, for PP-NEXAFS simulations the choice of the density functional is important. Especially hybrid functionals (which could not be used straightforwardly before to simulate PP-NEXAFS spectra) and their amount of "Hartree-Fock like" exact exchange affects relative resonance positions in the spectrum.}, language = {en} } @article{EhlertGuehrSaalfrank2018, author = {Ehlert, Christopher and G{\"u}hr, Markus and Saalfrank, Peter}, title = {An efficient first principles method for molecular pump-probe NEXAFS spectra}, series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, volume = {149}, journal = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, number = {14}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-9606}, doi = {10.1063/1.5050488}, pages = {13}, year = {2018}, abstract = {Pump-probe near edge X-ray absorption fine structure (PP-NEXAFS) spectra of molecules offer insight into valence-excited states, even if optically dark. In PP-NEXAFS spectroscopy, the molecule is "pumped" by UV or visible light enforcing a valence excitation, followed by an X-ray "probe" exciting core electrons into (now) partially empty valence orbitals. Calculations of PP-NEXAFS have so far been done by costly, correlated wavefunction methods which are not easily applicable to medium-sized or large molecules. Here we propose an efficient, first principles method based on density functional theory in combination with the transition potential and Delta SCF methodology (TP-DFT/Delta SCF) to compute molecular ground state and PP-NEXAFS spectra. We apply the method to n ->pi* pump/O-K-edge NEXAFS probe spectroscopy of thymine (for which both experimental and other theoretical data exist) and to n -> pi* or pi -> pi* pump/N-K-edge NEXAFS probe spectroscopies of trans-and cis-azobenzene. Published by AIP Publishing.}, language = {en} } @article{HeinrichEhlertHatteretal.2018, author = {Heinrich, Benjamin W. and Ehlert, Christopher and Hatter, Nino and Braun, Lukas and Lotze, Christian and Saalfrank, Peter and Franke, Katharina J.}, title = {Control of oxidation and spin state in a single-molecule junction}, series = {ACS nano}, volume = {12}, journal = {ACS nano}, number = {4}, publisher = {American Chemical Society}, address = {Washington}, issn = {1936-0851}, doi = {10.1021/acsnano.8b00312}, pages = {3172 -- 3177}, year = {2018}, abstract = {The oxidation and spin state of a metal-organic molecule determine its chemical reactivity and magnetic properties. Here, we demonstrate the reversible control of the oxidation and spin state in a single Fe porphyrin molecule in the force field of the tip of a scanning electron tunneling microscope. Within the regimes of half-integer and integer spin state, we can further track the evolution of the magnetocrystalline anisotropy. Our experimental results are corroborated by density functional theory and wave function theory. This combined analysis allows us to draw a complete picture of the molecular states over a large range of intramolecular deformations.}, language = {en} } @article{EhlertKlamroth2017, author = {Ehlert, Christopher and Klamroth, Tillmann}, title = {The quest for best suited references for configuration interaction singles calculations of core excited states}, series = {Journal of computational chemistry : organic, inorganic, physical, biological}, volume = {38}, journal = {Journal of computational chemistry : organic, inorganic, physical, biological}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0192-8651}, doi = {10.1002/jcc.24531}, pages = {116 -- 126}, year = {2017}, abstract = {Near edge X-ray absorption fine structure (NEXAFS) simulations based on the conventional configuration interaction singles (CIS) lead to excitation energies, which are systematically blue shifted. Using a (restricted) open shell core hole reference instead of the Hartree Fock (HF) ground state orbitals improves (Decleva et al., Chem. Phys., 1992, 168, 51) excitation energies and the shape of the spectra significantly. In this work, we systematically vary the underlying SCF approaches, that is, based on HF or density functional theory, to identify best suited reference orbitals using a series of small test molecules. We compare the energies of the K edges and NEXAFS spectra to experimental data. The main improvement compared to conventional CIS, that is, using HF ground state orbitals, is due to the electrostatic influence of the core hole. Different SCF approaches, density functionals, or the use of fractional occupations lead only to comparably small changes. Furthermore, to account for bigger systems, we adapt the core-valence separation for our approach. We demonstrate that the good quality of the spectrum is not influenced by this approximation when used together with the non-separated ground state wave function. Simultaneously, the computational demands are reduced remarkably. (C) 2016 Wiley Periodicals, Inc.}, language = {en} } @article{DietrichGlamschEhlertetal.2016, author = {Dietrich, Paul M. and Glamsch, Stephan and Ehlert, Christopher and Lippitz, Andreas and Kulak, Nora and Unger, Wolfgang E. S.}, title = {Synchrotron-radiation XPS analysis of ultra-thin silane films: Specifying the organic silicon}, series = {Applied surface science : a journal devoted to applied physics and chemistry of surfaces and interfaces}, volume = {363}, journal = {Applied surface science : a journal devoted to applied physics and chemistry of surfaces and interfaces}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0169-4332}, doi = {10.1016/j.apsusc.2015.12.052}, pages = {406 -- 411}, year = {2016}, abstract = {The analysis of chemical and elemental in-depth variations in ultra-thin organic layers with thicknesses below 5 nm is very challenging. Energy- and angle-resolved XPS (ER/AR-XPS) opens up the possibility for non-destructive chemical ultra-shallow depth profiling of the outermost surface layer of ultra-thin organic films due to its exceptional surface sensitivity. For common organic materials a reliable chemical in-depth analysis with a lower limit of the XPS information depth z(95) of about 1 nm can be performed. As a proof-of-principle example with relevance for industrial applications the ER/AR-XPS analysis of different organic monolayers made of amino- or benzamidosilane molecules on silicon oxide surfaces is presented. It is demonstrated how to use the Si 2p core-level region to non-destructively depth-profile the organic (silane monolayer) - inorganic (SiO2/Si) interface and how to quantify Si species, ranging from elemental silicon over native silicon oxide to the silane itself. The main advantage of the applied ER/AR-XPS method is the improved specification of organic from inorganic silicon components in Si 2p core-level spectra with exceptional low uncertainties compared to conventional laboratory XPS. (C) 2015 Elsevier B.V. All rights reserved.}, language = {en} } @article{SchmidtWolfEhlert2016, author = {Schmidt, Bernd and Wolf, Felix and Ehlert, Christopher}, title = {Systematic Investigation into the Matsuda-Heck Reaction of alpha-Methylene Lactones: How Conformational Constraints Direct the beta-H-Elimination Step}, series = {The journal of organic chemistry}, volume = {81}, journal = {The journal of organic chemistry}, publisher = {American Chemical Society}, address = {Washington}, issn = {0022-3263}, doi = {10.1021/acs.joc.6b02207}, pages = {11235 -- 11249}, year = {2016}, abstract = {alpha-Methylene-gamma-butyrolactone and alpha-methylene-gamma-valerolactone undergo Pd-catalyzed Matsuda-Heck couplings with arene diazonium salts to alpha-benzyl butenolides or pentenolides, respectively, or to alpha-benzylidene lactones. The observed regioselectivity is strongly ring size dependent, with six-membered rings giving exclusively alpha-benzyl pentenolides, whereas the five-membered alpha-methylene lactone reacts to mixtures of regioisomers with a high proportion of (E)-alpha-benzylidene-gamma-butyrolactones. DFT calculations suggest that the reasons for these differences are not thermodynamic but kinetic in nature. The relative energies of the conformers of the Pd sigma-complexes resulting from insertion into the Pd-aryl bond were correlated with the dihedral angles between Pd and endo-beta-H. This correlation revealed that in the case of the six-membered lactone an energetically favorable conformer adopts a nearly synperiplanar Pd/endo-beta-H arrangement, whereas for the analogous Pd sigma-complex of the five-membered lactone the smallest Pd/endo-beta-H dihedral angle is observed for a conformer with a comparatively high potential energy. The optimized conditions for Matsuda-Heck arylations of exo-methylene lactones were eventually applied to the synthesis of the natural product anemarcoumarin A.}, language = {en} } @article{EhlertHolzweberLippitzetal.2016, author = {Ehlert, Christopher and Holzweber, Markus and Lippitz, Andreas and Unger, Wolfgang E. S. and Saalfrank, Peter}, title = {A detailed assignment of NEXAFS resonances of imidazolium based ionic liquids}, series = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, volume = {18}, journal = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1463-9076}, doi = {10.1039/c5cp07434g}, pages = {8654 -- 8661}, year = {2016}, abstract = {In Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy X-Ray photons are used to excite tightly bound core electrons to low-lying unoccupied orbitals of the system. This technique offers insight into the electronic structure of the system as well as useful structural information. In this work, we apply NEXAFS to two kinds of imidazolium based ionic liquids ([C(n)C(1)im](+)[NTf2](-) and [C(4)C(1)im](+)[I](-)). A combination of measurements and quantum chemical calculations of C K and N K NEXAFS resonances is presented. The simulations, based on the transition potential density functional theory method (TP-DFT), reproduce all characteristic features observed by the experiment. Furthermore, a detailed assignment of resonance features to excitation centers (carbon or nitrogen atoms) leads to a consistent interpretation of the spectra.}, language = {en} }