Impulsive Stimulated X-ray Raman Scattering in the condensed phase with Attosecond XFEL Pulses

Charge migration (CM) refers to the coherent motion of electronic charge in molecular systems, typically exhibiting dynamics on attosecond timescales. This electronic motion, by coupling to other degrees of freedom in the system, can inform processes critical to chemistry. These secondary processes occur on much longer timescales, and examples include isomerisation, bond fragmentation and charge transfer. This abundance of subsequently influenced effects has motivated a focus on developing theoretical models and developing experimental techniques to characterize CM for different systems, ranging from gas phase to the condensed phase.

Impulsive stimulated X-ray Raman scattering (ISXRS) is predicted to generate and allow detection of spatially-localised valence-excited wavepackets in a site selective manner - allowing for CM to be measured between specific atomic sites in heteroatomic systems. We present the first evidence for ISXRS in the condensed-phase, by focusing isolated attosecond soft x-ray pulses at the chemRIXS endstation at the Linac Coherent Light Source X-ray Free Electron Laser with power densities up to 10¹⁷ Wcm^-2 on a water jet. Our observable is given by an enhancement of scattered modes of the x-ray field consistent with stimulated Stokes emission into the valence electronic excited states of the water molecules in the liquid phase.

We present calculations for the polarisation response of a water dimer following interaction with the x-ray field; hence allowing for the modelling of hydrogen bond interactions in bulk solvation environment. This results in good agreement with the experiment and offers insight into the dynamic populations of the core and valence-excited states during the process. We anticipate this work can serve as a building block for future time-resolved ISXRS pump-probe schemes, enabling characterization of CM in neutral condensed phase systems.