Enhanced nonlinear response of Ne⁸⁺ to intense ultrafast X-rays

Arina Sytcheva, Stefan Pabst, Sang-Kil Son, and Robin Santra

[BibTeX]http://www.dpg-verhandlungen.de/year/2012/conference/stuttgart/part/a/session/11/contribution/4

We report on the possible reasons for the discrepancy between the theoretical two-photon ionization cross section (TPICS),   4·10^-56 cm⁴ s, of Ne⁸⁺ obtained within the lowest nonvanishing order of perturbation theory (LOPT) for continuous light and the experimental value, 7×10^-54 cm⁴ s, reported in [Phys. Rev. Lett. 106, 083002 (2011)] at a photon energy of 1110 eV and pulse bandwidth of 11 eV. We consider Ne⁸⁺ exposed to coherent and chaotic ensembles of intense X-ray pulses using the time-dependent configuration-interaction singles (TDCIS) method and LOPT, respectively. A coherent ensemble of pulses of 11-eV bandwidth centered at 1110 eV yields a TPICS enhanced by a factor of ∼ 1.25 with respect to the continuous-light TPICS, while a chaotic ensemble yields an enhancement factor of ∼ 40. This enhancement is due to the presence of the one-photon 1s²–1s4p resonance located at 1127 eV and the finite bandwidth of the X-ray pulse. Using the TDCIS approach, we also show that, for currently available radiation intensities, two-photon ionization of a 1s electron in neutral neon is much less probable than one-photon ionization of a valence electron.