# Effective Quantum Theory of EXAFS in a Dissipative Liquid-Phase Medium

## TL;DR

This paper introduces a new quantum theory to model EXAFS spectra in liquids, using water as an example and showing good agreement with experiments.

## Contribution

A novel effective quantum theory for EXAFS in liquids using a complex dielectric function and GW approximation is proposed.

## Key findings

- The effective theory successfully recovers known inelastic mean free paths of photoelectrons in water.
- Theoretical results for bromide and chloride atoms in water match experimental data.
- Single-scattering processes dominate the EXAFS signal in the studied systems.

## Abstract

The extended X-ray
absorption fine structure (EXAFS) spectroscopy
is a powerful tool to determine the microscopic structure in the vicinity
of a probe atom or molecule embedded in a host material. For absorbing
atoms dissolved in a liquid, the disordered nature of the host poses
challenges for the theoretical calculation of the EXAFS spectrum,
especially when strong inelastic energy and momentum transfer between
the photoelectron and the solvent occurs. We formulate here an effective
quantum theory of EXAFS that is based on the use of an accurately
parametrized complex dielectric function of the solvent, illustrated
here for the case of water. We derive an effective complex self-energy
within the GW approximation to determine the EXAFS signal within a
single-scattering approach. To verify the approach, we recover the
results for the inelastic mean free path of a photoelectron in water,
as known in the literature. In addition, we apply this effective approach
to the case of single bromide and chloride atoms dissolved in water
and show that the theoretical results match available experimental
data. Through advanced FEFF simulations, which include accurate multiple-scattering
effects, we conclude that the contribution of the single-scattering
processes is dominant. We show that a key role is played by the dielectric
environment.