Simulating X-ray Absorption Spectroscopy Beyond the Dipole Approximation: A Relativistic Formulation. – (Martin van Horn / LCPQ / Thesis). – 24/11/2023, 9H

Thesis defence

Martin van Horn, LCPQ, Seminar room, Third floor 3R1,

Summary :
In the simulation of UV-Vis spectroscopy, multipole expansions are an indispensable tool to calculate absorption intensities, justified by the vast difference in scale between the wave length of light and the spatial extent of the molecular system. Depending on the type of spectroscopy, conventional or electronic circular dichroism, this expansion yields the electric-dipole approximation or the rotational strength, respectively, at first order.
I will use the term dipole approximation throughout this thesis as a single descriptor encompassing both schemes.
However, in the X-ray regime, where the wave length can reach molecular dimensions, this approximation comes into question.
The main goal of this thesis is to assess the validity of the dipole approximation in the simulation of X-ray absorption spectroscopy.
In the pursuit of this goal, a fully relativistic approach is employed to correctly describe the core electrons.
Furthermore, this approach allows us to reach the far ends of hard X-ray regime, typically probing core electrons of heavy elements.

In general, there are two methods to include non-dipolar effects: either the semi-classical light–matter interaction is treated exactly, or the multipole expansion is truncated beyond first order.
For the truncated approach there are two possible representations that are equivalent in the complete basis set limit: the generalized length- and velocity representation, where in the former, the multipoles are given in their conventional form known from classical electrodynamics.
Both of these schemes have been implemented in \dirac, a quantum chemistry code capable of two- and four-component relativistic calculations. In this thesis, the implementation of this code is discussed, together with four applications.