title: Multiphoton double ionization of Ar in intense extreme ultraviolet laser fields studied by shot-by-shot photoelectron spectroscopy.
abstract: Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field (hν ∼ 21  eV, ∼ 5  TW/cm²), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of Ar(+): Ar+hν→Ar (+) + e⁻ and Ar²(+) + 2hν→Ar(+) + e⁻. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from Ar(+) is significantly enhanced by intermediate resonances in Ar(+).
authors.affiliation: Department of Environmental Science, Niigata University, Niigata 950-2181, Japan. hikosaka@env.sc.niigata-u.ac.jp
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authors.lastname: Hikosaka
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authors.lastname: Matsuda
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authors.lastname: Tseng
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authors.lastname: Hishikawa
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authors.lastname: Shigemasa
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authors.lastname: Nagasono
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authors.lastname: Togashi
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authors.lastname: Ohashi
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authors.lastname: Kimura
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authors.lastname: Senba
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authors.lastname: Yabashi
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authors.lastname: Ishikawa