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Nonlinear atomic response to ultraintense and ultrashort x-ray pulses
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. Messerschmidt, D. A. Reis, N. Rohringer, L. Young, P. Agostini, and L. F. DiMauro75th Annual Meeting of the DPG and DPG Spring Meeting of the Section AMOP
(German Physical Society, Dresden, Germany, March 13-18, 2011) [oral presentation]
The nonlinear response of neon atoms to ultraintense, ultrashort x-ray pulses is investigated with the LCLS x-ray free-electron laser. The production of Ne9+ is observed at kilovolt x-ray photon energies below the absorption edge of the Ne8+ ground state and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of direct 2-photon ionization of the Ne8+ ground state and a high-order sequential process involving two 1-photon ionization processes via transient excited states on a time scale faster than the Auger decay. We find that the nonlinear 2-photon ionization cross section is orders of magnitude larger than expected.
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