Nonlinear atomic response to intense ultrashort x rays

• Published in: Physical review letters Vol. 106; no. 8; p. 083002
• Main Authors: Doumy, G, Roedig, C, Son, S-K, Blaga, C I, DiChiara, A D, Santra, R, Berrah, N, Bostedt, C, Bozek, J D, Bucksbaum, P H, Cryan, J P, Fang, L, Ghimire, S, Glownia, J M, Hoener, M, Kanter, E P, Krässig, B, Kuebel, M, Messerschmidt, M, Paulus, G G, Reis, D A, Rohringer, N, Young, L, Agostini, P, DiMauro, L F
• Format: Journal Article
• Language: English
• Published: United States 24.02.2011
• ISSN: 1079-7114
• DOI: 10.1103/physrevlett.106.083002

The nonlinear absorption mechanisms of neon atoms to intense, femtosecond kilovolt x rays are investigated. The production of Ne(9+) is observed at x-ray frequencies below the Ne(8+), 1s(2) 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 Ne(8+) 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.