Heterodyne-Detected Ultrafast X-Ray Diffraction and Scattering from Nonstationary States

TL;DR

This paper presents a comprehensive theoretical framework for heterodyne-detected ultrafast X-ray scattering from nonstationary molecular states, enabling detailed analysis of electronic coherences and structural dynamics in various phases.

Contribution

It introduces a unified description of heterodyne-detected X-ray scattering processes, clarifying conditions for observing holographic interference in nonstationary states.

Findings

Unified theoretical framework for heterodyne-detected X-ray scattering

Conditions for observing holographic interference clarified

Analysis applied to gas-phase NaI

Abstract

Free-electron laser hard X-ray light sources can provide high fluence, femtosecond pulses, enabling the time-resolved probing of structural dynamics and elementary relaxation processes in molecules. Traditional X-ray elastic scattering from crystals in the ground state consists of sharp Bragg diffraction peaks that arise from pairs of molecules and reveal the ground state charge density. Scattering of ultrashort X-ray pulses from gases, liquids, and even single molecules is more complex and involves both single- and two- molecule contributions, diffuse (non-Bragg) features, elastic and inelastic components, contributions of electronic coherences in nonstationary states, and interferences between scattering off different states (heterodyne detection). We present a unified description that covers all these processes and discuss their relative magnitudes for gas-phase NaI. Conditions for the observation of holographic (heterodyne) interference, which has been recently discussed [1], are clarified.