@conference{Sytcheva12, author={Arina Sytcheva and Stefan Pabst and Sang-Kil Son and Robin Santra}, title={Enhanced nonlinear response of {Ne}^{8+} to intense ultrafast X-rays}, year={2012}, month={March 12-16}, booktitle={DPG Spring Meeting of the Section AMOP}, organization={German Physical Society}, address={Stuttgart, Germany}, url={http://www.dpg-verhandlungen.de/year/2012/conference/stuttgart/part/a/session/11/contribution/4}, abstract={We report on the possible reasons for the discrepancy between the theoretical two-photon ionization cross section (TPICS), ~ 4$\cdot$10$^{-56}$ cm$^4$ s, of Ne$^{8+}$ obtained within the lowest nonvanishing order of perturbation theory (LOPT) for continuous light and the experimental value, 7$\times$10$^{-54}$ cm$^4$ s, reported in [Phys. Rev. Lett. \textbf{106}, 083002 (2011)] at a photon energy of 1110 eV and pulse bandwidth of 11 eV. We consider Ne$^{8+}$ 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 бн 1.25 with respect to the continuous-light TPICS, while a chaotic ensemble yields an enhancement factor of бн 40. This enhancement is due to the presence of the one-photon 1$s$$^2$–1$s$4$p$ 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 1$s$ electron in neutral neon is much less probable than one-photon ionization of a valence electron.
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}
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