Impulsive X-ray Raman in liquid water

We investigate the use of X-ray Raman scattering to excite valence states of neutral atoms in micrometre thick liquid water targets using attosecond XFEL pulses tuned below the oxygen K-edge and with intensity up to 10¹⁷ Wcm^-² . We measure transmitted X-ray spectra and observe nonlinear emission features approximately 10 eV below the edge. Our numerical calculations based on the polarisation response of molecular water and considering propagation (Maxwell’s equations) show that this emission is consistent with X-ray Raman sidebands. These calculations show that the valence excited states are populated by the X-ray pulse and the final state populations are affected by propagation effects, which may limit the thickness of condensed phase targets for pump-probe experiments. Excitation fractions are low (<0.1%) but increase at higher intensities, where impulsive X-ray Raman could be used to excite and measure valence wavepackets. This opens new avenues for the study of attosecond charge dynamics and the investigation of electron wavepackets for pump-probe experiments, which is typically restricted to cationic wavepackets created via strong-field or impulsive ionisation. Valence state populations are affected by propagation effects for several femtoseconds, in part due to reshaping of the X-ray pulses, which may limit the thickness of condensed matter targets.