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Enhanced nonlinear response of Ne^{8+} to intense ultrafast x raysArina Sytcheva, Stefan Pabst, Sang-Kil Son, and Robin Santra Phys. Rev. A 85, 023414 (2012)[BibTeX][pdf]doi:10.1103/PhysRevA.85.023414 We investigate the possible reasons for the discrepancy between the theoretical two-photon ionization cross section, ∼10^{-56} cm^{4}s, of Ne^{8+} obtained within the perturbative nonrelativistic framework for monochromatic light [S. Novikov and A. Hopersky, J. Phys. B 34, 4857 (2001)] and the experimental value, 7×10^{-54} cm^{4}s, reported in [G. Doumy /textitet al., Phys. Rev. Lett. 106, 083002 (2011)] at a photon energy of 1110 eV. To this end, we consider Ne^{8+} exposed to deterministic and chaotic ensembles of intense x-ray pulses. The time-dependent configuration-interaction singles (TDCIS) method is used to quantitatively describe nonlinear ionization of Ne^{8+} induced by coherent intense ultrashort x-ray laser pulses. The impact of the bandwidth of a chaotic ensemble of x-ray pulses on the effective two-photon ionization cross section is studied within the lowest nonvanishing order of perturbation theory. We find that, at a bandwidth of 11 eV, the effective two-photon ionization cross section of Ne^{8+} at a photon energy of 1110 eV amounts to 5×10^{-57} and 1.6×10^{-55} cm^{4}s for a deterministic ensemble and a chaotic ensemble, respectively. We show that the enhancement obtained for a chaotic ensemble of pulses originates from the presence of the one-photon 1s^{2}–1s4p resonance located at 1127 eV. Using the TDCIS approach, we also show that, for currently available radiation intensities, two-photon ionization of a 1s electron in neutral neon remains less probable than one-photon ionization of a valence electron. Tags: Ne, x-ray, nonlinear, two-photon, cross section, TDCIS, XATOM, XCID, XFEL, CFEL, DESY |
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