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Abstract
This Ph.D. thesis explores different theoretical methods to describe chemical processes via Xray spectroscopy with an emphasis on Xray absorption spectroscopy (XAS). This includes method development, comparisons of methods, and analysis of experimental data. The thesis reviews the theory for describing electronic structure and nuclear dynamics, and the properties of interest are presented. These rely on a single molecular geometry, which is determined by using a molecular gradient. An efficient implementation of such a gradient has been carried out as part of the presented work at the coupled cluster singles and doubles (CCSD) level of theory for both ground and excited states. Thus, the corresponding equilibrium structures can be determined. Also, the implementation lays the ground work for describing nuclear dynamics at the CCSD level of theory. Nuclear dynamics is here simulated with timedependent density functional theory (TDDFT) using, e.g., trajectory surface hopping (TSH), where nuclei are treated with classical mechanics. This gives a qualitatively good description of the nuclear dynamics, even for a limited number of trajectories. A TSH calculation is also found to be a good basis for a quantum dynamics simulation. The CCSD method was employed to calculate different static Xray spectra. It showed an overall good agreement with experiment for, e.g., XAS, Xray photoelectron spectroscopy and resonant inelastic Xray scattering. This indicates that the method yields a good description of the studied processes. The CCSD method was, for instance, used to analyse experimental ground state spectra of the heterocycle oxazole. This is a first step towards analyzing future timeresolved XAS (trXAS) experiments of the system. In the present study, TDDFT was also employed to calculate XAS spectra. Here, this less computationally expensive method turned out to yield results of similar accuracy to CCSD. This was also noticed in the analysis of experimental trXAS data for a larger molecule. Moreover, simulations of trXAS based on the above mentioned methods, showed that for rigid molecules, trXAS can be simulated based on optimized geometries alone. Indeed, when the molecule remains in the same electronic state for a long time, one might even avoid a nuclear dynamics simulation. Several states of interest will, however, require such a simulation to predict the evolution of state populations. These conclusions must be further investigated for less rigid molecules.
Original language  English 

Publisher  DTU Chemistry 

Number of pages  390 
Publication status  Published  2022 
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Dive into the research topics of 'On Simulating Ultrafast Chemical Processes and their Spectroscopic Signatures'. Together they form a unique fingerprint.Projects
 1 Finished

Development and application of theoretical methodologies for the comprehensive description and interpretation of timeresolved spectroscopic and scattering experiments in the femtoand attosecond time scale
SchnackPetersen, A. K. (PhD Student), Doslic, N. (Examiner), Møller, K. B. (Main Supervisor), Coriani, S. (Supervisor), Koch, H. (Supervisor) & Hättig, C. (Examiner)
01/11/2019 → 01/03/2023
Project: PhD