Impulsive Stimulated X-ray Raman Scattering in Molecular Systems

The field of nonlinear X-ray interactions is growing with technological advances at free-electron lasers. The development of intense attosecond X-ray pulses has opened a new regime for studying nonlinear X-ray interactions in the impulsive limit. In an experiment conducted at the Linac Coherent Light Source, we made the first observation of impulsive stimulated X-ray Raman scattering~(ISXRS) in a molecular system, finding a two-photon cross section of~$\left(3\pm2\right)\times10^{-55}$~cm$^4$s/photon. ISXRS is a versatile process for studying electronic charge motion and X-ray-matter interactions. Simulations matching the observed signal show that ISXRS coherently populates multiple valence-excited electronic states. This is a significant step toward direct observation of coherent electronic wavepackets using the planned 2-color attosecond X-ray mode of the LCLS.