X-ray nonlinear optical processes using a self-amplified spontaneous emission free-electron laser

In contrast to the long-wavelength regime, x-ray nonlinear optical processes are characterized in general by sequential single-photon single-electron interactions. Despite this fact, the sequential absorption of multiple x-ray photons depends on the statistical properties of the radiation field. Treating the x rays generated by a self-amplified spontaneous emission free-electron laser as fully chaotic, a quantum-mechanical analysis of inner-shell two-photon absorption is performed. It is demonstrated that double-core-hole formation via x-ray two-photon absorption is enhanced by chaotic photon statistics. Numerical calculations using rate equations illustrate the impact of field chaoticity on x-ray nonlinear ionization of helium and neon for photon energies near 1 keV. In the case of neon, processes are discussed that involve up to seven photons. Assuming an x-ray coherence time of 2.6 fs, double-core-hole formation in neon is found to be statistically enhanced by about 30% at an intensity of 10(16) W/cm(2).