Enhanced nonlinear response of Ne 8+ to intense ultrafast x rays

Abstract

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 and the experimental value, 7×10 -54 cm4s, reported in 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. © 2012 American Physical Society.