A time-correlation function approach to nuclear dynamical effects in X-ray spectroscopy

TL;DR

This paper introduces a novel time-correlation function approach to incorporate nuclear dynamical effects into X-ray spectroscopy, enhancing the understanding of nuclear-electronic interactions in complex environments.

Contribution

It presents an alternative derivation and implementation of a protocol to account for nuclear dynamics in X-ray spectra, demonstrated on water samples.

Findings

Nuclear motions influence specific spectral features.

Correlation functions reveal dynamics of electronic energy gaps.

Harmonic model explains observed spectral tendencies.

Abstract

Modern X-ray spectroscopy has proven itself as a robust tool for probing the electronic structure of atoms in complex environments. Despite working on energy scales that are much larger than those corresponding to nuclear motions, taking nuclear dynamics and the associated nuclear correlations into account may be of importance for X-ray spectroscopy. Recently, we have developed an efficient protocol to account for nuclear dynamics in X-ray absorption and resonant inelastic X-ray scattering spectra [Karsten \textit{et al.} arXiv:1608.03436], based on ground state molecular dynamics accompanied with state-of-the-art calculations of electronic excitation energies and transition dipoles. Here, we present an alternative derivation of the formalism and elaborate on the developed simulation protocol on the examples of gas phase and bulk water. The specific spectroscopic features stemming from the nuclear motions are analyzed and traced down to the dynamics of electronic energy gaps and transition dipole correlation functions. The observed tendencies are explained on the basis of a simple harmonic model and the involved approximations are discussed. The method represents a step forward over the conventional approaches treating the system in full complexity and provides a reasonable starting point for further improvements.