X-Ray sum frequency generation; direct imaging of ultrafast electron dynamics

TL;DR

This paper introduces a novel nonlinear X-ray technique using free electron lasers to directly image ultrafast valence electron dynamics, overcoming limitations of traditional diffraction methods.

Contribution

The study presents a new sum frequency generation method with X-ray and visible pulses for imaging transition charge densities in excited electrons.

Findings

Successfully simulated transition charge density imaging.

Demonstrated ability to monitor ultrafast electron dynamics.

Provided a new approach for observing electronic excitations.

Abstract

X-ray diffraction from molecules in the ground state produces an image of their charge density, and time-resolved X-ray diffraction can thus monitor the motion of the nuclei. However, the density change of excited valence electrons upon optical excitation can barely be monitored with regular diffraction techniques due to the overwhelming background contribution of the core electrons. We present a nonlinear X-ray technique made possible by novel free electron laser sources, which provides a spatial electron density image of valence electron excitations. The technique, sum frequency generation carried out with a visible pump and a broadband X-ray diffraction pulse, yields snapshots of the transition charge densities, which represent the electron density variations upon optical excitation. The technique is illustrated by ab initio simulations of transition charge density imaging for the optically induced electronic dynamics in a donor/acceptor substituted stilbene.