Nonlinear atomic response to intense ultrashort x rays

Gilles Doumy, C. Roedig, Sang-Kil Son, C. I. Blaga, A. D. DiChiara, Robin Santra, N. Berrah, C. Bostedt, J. D. Bozek, P. H. Bucksbaum, J. P. Cryan, L. Fang, S. Ghimire, J. M. Glownia, M. Hoener, E. P. Kanter, B. Krässig, M. Kuebel, M. Messerschmidt, G. G. Paulus, D. A. Reis, N. Rohringer, L. Young, P. Agostini, and L. F. DiMauro

[BibTeX][pdf]doi:10.1103/PhysRevLett.106.083002

The nonlinear absorption mechanisms of neon atoms to intense, femtosecond kilovolt x rays are investigated. The production of Ne⁹⁺ is observed at x-ray frequencies below the Ne⁸⁺, 1s² absorption edge and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of the two-photon ionization of Ne⁸⁺ ground state and a high-order sequential process involving single-photon production and ionization of transient excited states on a time scale faster than the Auger decay. We find that the nonlinear direct two-photon ionization cross section is orders of magnitude higher than expected from previous calculations.

Tags: Ne, nonlinear, x-ray, LCLS, experiment, XATOM, XFEL, CFEL, DESY