Enhanced nonlinear response of Ne8+to intense ultrafast x rays

Abstract

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 et 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.

Nonsequential two-photon absorption from the K shell in solid zirconium

“…We note that He experiments were performed above the onephoton threshold where it competes with two-photon double ionization. The He-like Ne results were also measured far from the two-photon threshold, and the relative difference from perturbative scaling was attributed to the contributions from near-resonant states [14]. In the current work on Zr, the upper (lower) limit of the cross section is about 23 (1.3) times higher than the Z −6 scaled results for Z = 40.…”

“…The results were found to be 2-3 orders of magnitude higher than both the nonresonant perturbative scaling of Ref. [11] and the second-order perturbation theory calculations of Novikov and Hopersky [13], and were attributed to contributions from near-resonant states [14].…”

Compton spectra of atoms at high x-ray intensity

“…Firstly, xatom has served as an essential toolkit for various XFEL experiments on gas-phase atoms [14,39,40,41,42] conducted at LCLS [43] and at SACLA [44]. It has provided capabilities to interpret experimental data, to examine nonlinear two-photon x-ray ionization [14,45], and to propose a novel x-ray ionization mechanism involving resonant excitations and accompanying decays [39,41]. The noble gas results obtained by xatom have been used to calibrate x-ray beam profiles [39,40,46,47].…”

Resonance-enhanced multiple ionization of krypton at an x-ray free-electron laser

“…It was demonstrated that all electrons from light elements like neon or nitrogen can be stripped in a single pulse provided the photon energy is higher than all ionization potentials of the intermediate ionic states. The rate of direct ("nonsequential") two-photon ionization has been shown to be small [6,7] and its contribution to the total ionization yield can be neglected whenever sequential ionization is energetically allowed. Also, hidden resonant excitations were unveiled in high charge states of neon [8], but their role in the overall photoionization process is negligible for light atoms as they appear very localized.…”