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

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.