Anomalous two-photon Compton scattering

Abstract

X-ray free-electron lasers can generate radiation pulses with extreme peak intensities at short wavelengths. This enables the investigation of laser–matter interactions in a regime of high fields, yet at a non-relativistic ponderomotive potential, where ordinary rules of light–matter interaction may no longer apply and nonlinear processes are starting to become observable. Despite small cross-sections, first nonlinear effects in the hard x-ray regime have recently been observed in solid targets, including x-ray-optical sum-frequency generation (XSFG), x-ray second harmonic generation (XSHG) and two-photon Compton scattering (2PCS). Nonlinear interactions of bound electrons in the x-ray range are fundamentally different from those dominating at optical frequencies. Whereas in the optical regime nonlinearities are predominantly caused by anharmonicities of the atomic potential in the chemical bonds, x-ray nonlinearities far above atomic resonances are expected to be due to nonlinear oscillations of quasi-free electrons, including inner-shell atomic electrons. While the quasi-free-electron model agrees reasonably well with the experimental data for XSFG and XSHG, 2PCS measurements have led to unexpected results: the energy of the nonlinearly scattered photons from non-relativistic electrons shows a substantial unexpected red shift in addition to the Compton shift that is well beyond that predicted by a nonlinear quantum electrodynamics model for free electrons. A potential explanation for the spectral broadening is based on a previously unexplored scattering process that involves the whole atom rather than just quasi-free electrons. A first simulation that includes the atomic binding potential was successful in describing a broadening of the spectrum of the nonlinearly scattered photons to longer wavelengths for soft x-rays. However, the same model does not show any broadening at hard x-ray wavelengths, which is in agreement with other simulation approaches. To this point no calculation has been able to reproduce the experimentally observed broadening. Here we present further experimental data of 2PCS for an extended parameter range using additional diagnostics. In particular, we present measurements of the electron momentum distribution during the interaction that strongly suggest that the spectral broadening is not caused by an increased plasma temperature. We extend our measurement of the magnitude of the red shift in beryllium to > 1.9 k e V in addition to the Compton shift expected for free electrons and expand the measurement of the angular distribution to include forward scattering angles. We also present first measurements of 2PCS from diamond.

Numerical Simulation of Heat Load for Multilayer Laue Lens under Exposure to XFEL Pulse Trains

“…When the nodes of this standing wave are located in the more absorbing layers, then the beam is attenuated less than expected from Equation (2), known as the Bormann effect [25]. In such a condition, less energy is absorbed in the structure than predicted by Equation (2). In crystals, this occurs when the structure is oriented relative to the incident beam to produce the highest possible reflected intensity [25] but is not necessarily the case for multilayer structures [29].…”

“…The short wavelengths and short durations of XFEL pulses mean that these studies can be performed with a nanometer spatial resolution and femtosecond temporal resolution. Two-photon processes such as nonlinear Compton scattering [1] or two-photon absorption [2] are dependent on the square of the X-ray intensity and thus are best observed at the highest intensities (high numbers of photons per area and time). High intensities are also needed for single-molecule diffractive imaging [3,4], generating stimulated X-ray emission [5], and for utilizing the interference of fluorescence photons for imaging [6].…”