Stimulated X-ray Raman Spectroscopy with Free-Electron Lasers

Abstract:  X-ray free electron lasers (XFELs), with widely tunable output pulses possessing joint temporal and spatial resolution on the atomic scale and peak intensities up to 10 20 W/cm 2 , have revolutionized x-ray science. These characteristics have enabled multiphoton physics in the x-ray regime and the observation of nonlinear phenomena such as atomic x-ray lasing, second harmonic generation and optical/x-ray wavemixing. Going beyond these demonstrations, XFEL experiments have been proposed as a site-specific probe of ultrafast electronic excitations which can inform a wide variety of processes on their natural timescales, e.g. photosynthesis, and light harvesting. Here the use of nonlinear two-photon x-ray Raman techniques enables one to beat the core-hole lifetime that limits standard x-ray absorption and emission methods. In analogy with optical coherent Raman scattering techniques that probe ultrafast vibrational processes, x-ray nonlinear Raman techniques probe ultrafast electronic processes. Building from two pillars, propagation in dense media to amplify spontaneous Raman signals by ~10^8 and narrowband spectral spikes inherent in self-amplified spontaneous emission (SASE) pulses, we demonstrate high-resolution stimulated x-ray Raman spectroscopy (bandwidths ~0.3 eV) using broadband SASE pulses (~7 eV bandwidth). This ability to use broadband SASE pulses to achieve high-resolution, ultrafast x-ray Raman spectral snapshots allows one to rapidly acquire data in parallel and paves the way toward an understanding of site-specific electronic dynamics.

Work supported by U.S. Department of Energy, Office of Science, Basic Energy Science, Chemical Sciences, Geosciences and Biosciences Division under contract number DE-AC02-06CH11357.