Nonlinear light scattering in molecules triggered by an impulsive X-ray Raman process

TL;DR

This paper models nonlinear light scattering signals from molecules excited by impulsive X-ray pulses, revealing both coherent and incoherent components with distinct scaling behaviors.

Contribution

It introduces a superoperator Green's function formalism to calculate time- and frequency-resolved nonlinear light scattering signals in molecules after impulsive X-ray excitation.

Findings

Coherent signal scales as N^2, incoherent as N.

Coherent component follows classical Larmor formula.

Incoherent component involves quantum electronic structure details.

Abstract

The time-and-frequency resolved nonlinear light scattering (NLS) signals from a time evolving charge distribution of valence electrons prepared by impulsive X-ray pulses are calculated using a superoperator Green's function formalism. The signal consists of a coherent $\sim N^2$-scaling difference frequency generation and an incoherent fluorescence $\sim N$-scaling component where $N$ is the number of active molecules. The former is given by the classical Larmor formula based on the time-dependent charge density. The latter requires additional information about the electronic structure and may be recast in terms of transition amplitudes representing quantum matter pathways.