# Monitoring Nonadiabatic Avoided Crossing Dynamics in Molecules by Ultrafast X-Ray Diffraction

## TL;DR

This paper demonstrates how ultrafast X-ray diffraction can monitor nonadiabatic avoided crossing dynamics in molecules, revealing electronic and nuclear structural changes with detailed signal contributions and simulations of sodium fluoride.

## Contribution

It introduces a method to distinguish different scattering contributions in time-resolved X-ray diffraction during nonadiabatic dynamics, including electronic coherences and transition charge densities.

## Key findings

- Elastic scattering reveals molecular geometry changes.
- Electronic coherences contribute weaker but spatially informative signals.
- Simulations of sodium fluoride's nonadiabatic process validate the approach.

## Abstract

We examine time-resolved X-ray diffraction from molecules in the gas phase which undergo nonadiabatic avoided-crossing dynamics involving strongly coupled electrons and nuclei. Several contributions to the signal are identified, representing (in decreasing strength) elastic scattering, contributions of the electronic coherences created by nonadiabatic couplings in the avoided crossing regime, and inelastic scattering. The former probes the charge density and delivers direct information on the evolving molecular geometry. The latter two contributions are weaker and carry spatial information of the transition charge densities (off-diagonal elements of the charge-density operator). Simulations are presented for the nonadiabatic harpooning process in the excited states of sodium fluoride.