Anomalous two-photon Compton scattering

Journal Article · Thu Nov 18 00:00:00 EST 2021 · New Journal of Physics

DOI:https://doi.org/10.1088/1367-2630/ac3553· OSTI ID:1831310

Kettle, B. ; Aquila, A.; Boutet, S.; Bucksbaum, P. H.; Carini, G.; Feng, Y.; Gamboa, E.; Ghimire, S.; Glenzer, S.; Hart, P.; Hastings, J. B.; Henighan, T.; Hunter, M.; Koglin, J.; Kozina, M.; Liu, H.; MacDonald, M. J.; Trigo, M.; Reis, D. A.; Fuchs, M.

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 $>1.9\mathrm{k}\mathrm{e}\mathrm{V}$ 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.

• Research Organization: SLAC National Accelerator Lab., Menlo Park, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES); USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division
• Grant/Contract Number: AC02-76SF00515; SC0016494
• OSTI ID: 1831310
• Alternate ID(s): OSTI ID: 1828447; OSTI ID: 1846411
• Journal Information: New Journal of Physics, Journal Name: New Journal of Physics Vol. 23 Journal Issue: 11; ISSN 1367-2630
• Publisher: IOP Publishing
• Country of Publication: United Kingdom
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
nonlinear x-ray optics
nonlinear Compton scattering
strong-field physics