Nonlinear resonant X-ray Raman scattering

TL;DR

This paper reports the discovery of a nonlinear resonant X-ray Raman scattering effect in metals, where intense X-ray pulses induce two-photon excitation and generate high-energy photons with complex spectral features.

Contribution

It presents the first observation of nonlinear resonant X-ray Raman scattering involving double-core hole excitation and multielectron transitions in metals.

Findings

Photon emission scales quadratically with intensity.

Spectral lines disperse with incident photon energy below the double K-shell ionization threshold.

Above the threshold, lines become fluorescence lines with higher energies.

Abstract

We report the observation of a novel nonlinear effect in the hard x-ray range. Upon illuminating Fe and Cu metal foils with intense x-ray pulses tuned near their respective K edges, photons at nearly twice the incoming photon energy are emitted. The signal rises quadratically with the incoming intensity, consistent with two-photon excitation. The spectrum of emitted high-energy photons comprises multiple Raman lines that disperse with the incident photon energy. Upon reaching the double K-shell ionization threshold, the signal strength undergoes a marked rise. Above this threshold, the lines cease dispersing, turning into orescence lines with energies much greater than obtainable by single electron transitions, and additional Raman lines appear. We attribute these processes to electron-correlation mediated multielectron transitions involving double-core hole excitation and various two-electron de-excitation processes to a final state involving one or more L and M core-holes.