High temporal and spectral resolution of stimulated x-ray Raman signals with stochastic free-electron-laser pulses

TL;DR

This paper introduces a stochastic correlation technique for stimulated x-ray Raman spectroscopy that overcomes the limitations of chaotic free-electron-laser pulses, enabling high-resolution observation of molecular dynamics without phase control.

Contribution

The authors demonstrate a method to achieve high temporal and spectral resolution in x-ray Raman signals using stochastic pulse statistics, eliminating the need for spectral phase control.

Findings

Resolved ultrafast coherences in molecular systems.

Simulated joint temporal and spectral resolution for a molecular model.

Effective application to noisy free-electron-laser pulses.

Abstract

The chaotic nature of x-ray free-electron-laser pulses is a major bottleneck that has limited the joint temporal and spectral resolution of spectroscopic measurements. We show how to use the stochastic x-ray field statistics to overcome this difficulty by correlation signals averaged over independent pulse realizations. No control is required over the spectral phase of the pulse, enabling immediate application with existing, noisy x-ray free-electron-laser pulses. The proposed stimulated Raman technique provides the broad observation bandwidth and high time-frequency resolution needed for the observation of elementary molecular events. A model is used to simulate chaotic free-electron-laser pulses and calculate their correlation properties. The resulting joint temporal/spectral resolution is exemplified for a molecular model system with time-dependent frequencies and for the RNA base Uracil passing through a conical intersection. Ultrafast coherences, which represent a direct signature of the nonadiabatic dynamics, are resolved. The detail and depth of physical information accessed by the proposed stochastic signal are virtually identical to those obtained by phase-controlled pulses.