Photon-recoil imaging: A new route to study nonlinear x-ray physics

With the advent of XFEL radiation it has been a central goal to merge the merits of x-ray spectroscopy, especially its ability to probe atom-specific local electron dynamics, with nonlinear techniques developed in the optical regime. Among the important nonlinear x-ray phenomena is the fundamental stimulated x-ray Raman scattering (SXRS) process near an inner-shell resonance, which is crucial for the development of any nonlinear x-ray method. Stimulated emission and stimulated Raman scattering have been observed in dense media by measuring the scattered x-ray radiation, and in molecules, where the final excited states have been detected via photoionization. In both cases, it is cumbersome to separate SXRS from spontaneous Raman processes.  Moreover, for x-ray detection the stimulated light is inherently emitted in the forward direction coinciding with the driving x-ray laser beam, identification of stimulated Raman scattering using x-ray detection techniques is per se largely impeded.

In this talk I will introduce photon-recoil imaging (PRI) as a new route to explore nonlinear x-ray physics. The idea of the method is to detect scattered neutral atoms rather than scattered x-rays. I will present our recent experiment at the European XFEL where we used the core resonances of neon as a showcase to demonstrate PRI for SXRS on a single-atom level. PRI allows for distinguishing the spontaneous x-ray Raman scattering, which is accompanied by a large momentum transfer to the atom, from SXRS, where momentum transfer is almost negligible. To gain access to the photon momentum transfer to the atom the deflection of scattered atoms in a collimated supersonic beam is measured. The experimental results agree very nicely with the results from a density matrix calculation using simulated XFEL pulses.

Finally, I will discuss the prospect of applying the method to molecules and possible experiments using the recently available two-color mode at XFELs, which allow for well-defined and time-controlled SXRS experiments, for which photon-recoil imaging will be particularly fruitful.