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Enhanced nonlinear response of Ne8+ to intense ultrafast x rays
Arina Sytcheva, Stefan Pabst, Sang-Kil Son, and Robin SantraPhys. Rev. A 85, 023414 (2012)
We investigate the possible reasons for the discrepancy between the theoretical two-photon ionization cross section, ∼10-56 cm4s, of Ne8+ 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 cm4s, reported in [G. Doumy /textitet al., Phys. Rev. Lett. 106, 083002 (2011)] at a photon energy of 1110 eV. To this end, we consider Ne8+ 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 Ne8+ 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 Ne8+ at a photon energy of 1110 eV amounts to 5×10-57 and 1.6×10-55 cm4s 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 1s2–1s4p 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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