X-ray spectroscopies through damped linear response theory
Abstract: In order to reach a fundamental understanding of interactions between electromagnetic radiation and molecular materials, experimental measurements need to be supplemented with theoretical models and simulations. With the use of this combination, it is possible to characterize materials in terms of, e.g., chemical composition and molecular structure, as well as achieve time-resolution in studies of chemical reactions. This doctoral thesis focuses on the development and evaluation of theoretical methods with which, amongst others, X-ray absorption and X-ray emission spectroscopies can be interpreted and predicted. In X-ray absorption spectroscopy the photon energy is tuned such that core electrons are targeted and excited to either bound or continuum states, and X-ray emission spectroscopy measures the subsequent decay from such an excited state. These core excitations/de-excitations exhibit strong relaxation effects, making theoretical considerations of the processes particularly challenging. While the removal of a valence electron leaves the remaining electrons relatively unaffected, removing core electrons has a substantial effect on the other electrons due to the significant change in the screening of the nucleus. Additionally, the core-excited states are embedded in a manifold of valence-excited states that needs to be considered by some computationally feasible method. In this thesis, a damped formalism of linear response theory, which is a perturbative manner of considering the interactions of (weak) external or internal fields with molecular systems, has been utilized to investigate mainly the X-ray absorption spectra of small- to medium-sized molecular systems.Amongst the standard quantum chemical methods available, coupled cluster is perhaps the most accurate, with a well-defined, hierarchical manner of approaching the correct electronic wave function. Combined with response theory, it provides a reliable theoretical method in which relaxation effects are addressed by means of an accurate treatment of electron correlation. The first part of this thesis deals with the development and evaluation of such an approach, and it is shown that the relaxation effects can be addressed by the inclusion of double excitations in the coupled cluster manifold. However, these calculations are computationally very demanding, and in order to treat larger systems the performance of the coupled cluster approach has been compared to that of the less demanding method of time-density dependent functional theory (TDDFT). Both methods have been used to investigate the X-ray absorption spectrum of water, which has been extensively debated in the scientific community following a relatively recent hypothesis concerning the underlying structure of liquid water. Water exhibits a great number of anomalous properties that stand out from those of most compounds, and the importance of reaching a fundamental understanding of this substance cannot be overstated. It has been demonstrated that TDDFT yields excellent results for liquid water, opening up possibilities of investigating the correlation between spectral features and local structures.Furthermore, recent developments in damped linear response TDDFT in the four-component relativistic regime have enabled the inclusion of spin-orbit coupling in damped linear response calculations, making black-box calculations of absorption spectra in a relativistic setting practical. With this approach, it is possible to address the spin-orbit splitting in L2,3-edge X-ray absorption spectra, and the performance of such a method has been demonstrated for a set of small molecules. Excellent agreement with experiment is obtained in terms of relative features, but an anomalous error in absolute energy has been observed for silane derivatives featuring fluorine-substitutions. This is likely a result of the strong influence of the very electronegative fluorine atoms on the electron density of the core-excited atom.Finally, the treatment of non-resonant X-ray emission spectroscopy using damped linear response theory is discussed. The expansion needed for the development of a simple method by which this spectroscopy can be treated using damped linear response theory at the TDDFT level of theory has been identified, and proof of principle calculations at the time-dependent Hartree-Fock level of theory are presented.
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