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Enhanced nonlinear double excitation of He in intense extreme ultraviolet laser fields
A Hishikawa1, M Fushitani, Y Hikosaka
1Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya, Aichi 464-8602, Japan. hishi@chem.nagoya-u.ac.jp
Abstract:
Nonlinear, three-photon double excitation of He in intense extreme ultraviolet free-electron laser fields (∼24.1 eV, ∼5 TW/cm2) is presented. Resonances to the doubly excited states converging to the He+ N=3 level are revealed by the shot-by-shot photoelectron spectroscopy and identified by theoretical calculations based on the time-dependent Schrödinger equation for the two-electron atom under a laser field. It is shown that the three-photon double excitation is enhanced by intermediate Rydberg states below the first ionization threshold, giving a greater contribution to the photoionization yields than the two-photon process by more than 1 order of magnitude.
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