title: Enhanced nonlinear double excitation of He in intense extreme ultraviolet laser fields.
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.
authors.affiliation: Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya, Aichi 464-8602, Japan. hishi@chem.nagoya-u.ac.jp
authors.forename: A
authors.initials: A
authors.lastname: Hishikawa
authors.forename: M
authors.initials: M
authors.lastname: Fushitani
authors.forename: Y
authors.initials: Y
authors.lastname: Hikosaka
authors.forename: A
authors.initials: A
authors.lastname: Matsuda
authors.forename: C-N
authors.initials: CN
authors.lastname: Liu
authors.forename: T
authors.initials: T
authors.lastname: Morishita
authors.forename: E
authors.initials: E
authors.lastname: Shigemasa
authors.forename: M
authors.initials: M
authors.lastname: Nagasono
authors.forename: K
authors.initials: K
authors.lastname: Tono
authors.forename: T
authors.initials: T
authors.lastname: Togashi
authors.forename: H
authors.initials: H
authors.lastname: Ohashi
authors.forename: H
authors.initials: H
authors.lastname: Kimura
authors.forename: Y
authors.initials: Y
authors.lastname: Senba
authors.forename: M
authors.initials: M
authors.lastname: Yabashi
authors.forename: T
authors.initials: T
authors.lastname: Ishikawa