Ultrafast anisotropic x-ray scattering in the condensed phase

T J Penfold,R Abela,Ursula Rothlisberger,I Tavernelli,M Chergui

doi:10.1088/1367-2630/14/11/113002

Abstract

The advent of x-ray free electron lasers offers new opportunities for x-ray scattering (XRS) studies of ultrafast molecular dynamics in liquids, which have so far been limited to the 100 ps resolution of synchrotrons. In particular, anisotropic XRS induced by photoselection, using a linearly polarized pump pulse, can enhance the contrast of the signal from excited molecules against the diffuse background and allows the probing of their vibrational and rotational dynamics. Here, we present a computational approach for calculating transient scattering intensities, based on molecular dynamics simulations. This is applied to the study of the excited state dynamics of molecular iodine dissolved in n-hexane. We report that at short times the transient XRS patterns reflect the evolving vibrational and rotational dynamics of I2, even when the disordered solvent environment is included. We then use these simulations to derive the anticipated signal-to-noise ratio for a large class of model diatomic systems in solution, indicating that an S/N ⩾1 will be possible from single-shot experiments in weakly scattering solvents.

Highlights

The advent of x-ray free electron lasers offers new opportunities for x-ray scattering (XRS) studies of ultrafast molecular dynamics in liquids, which have so far been limited to the 100 ps resolution of synchrotrons
Using a fs pump pulse of 520 nm with a spectral width of 10 nm, corresponding to a pulse width of ∼90 fs, we generate a wavepacket on the B state and obtain snapshots of the dynamics which are used to calculate the XRS patterns
The excited molecules form an anisotropic ensemble, and this is reflected in the fringe pattern of the transient scattering intensity that is visible in figure 2(a)

Summary

Introduction

The advent of x-ray free electron lasers offers new opportunities for x-ray scattering (XRS) studies of ultrafast molecular dynamics in liquids, which have so far been limited to the 100 ps resolution of synchrotrons. Equation (3) shows how the intensity can be calculated from snapshots of MD simulations of the excited molecular systems in solution. Our purpose in the present work is to translate into scattering patterns the structural (wavepacket) dynamics, occurring in a diatomic molecule.

Results

Conclusion