Photoelectron angular distributions for the two-photon ionization of helium by ultrashort extreme ultraviolet free-electron laser pulses

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The two-photon ionization of helium atoms by ultrashort extreme-ultraviolet free-electron laser pulses, produced by the SPring-8 Compact SASE Source test accelerator, was investigated at photon energies of 20.3, 21.3, 23.0 and 24.3 eV. The angular distribution of photoelectrons generated by two-photon ionization is obtained using a velocity map imaging spectrometer. The phase-shift differences and amplitude ratios of the outgoing s and d continuum wave packets are extracted from the photoelectron angular distributions. The obtained values of the phase-shift differences are distinct from scattering phase-shift differences when the photon energy is tuned to a resonance with an excited level or Rydberg manifold. The difference stems from the co-presence of resonant and non-resonant path contributions in the two-photon ionization by femtosecond pulses. Since the relative contribution of both paths can be controlled in principle by the pulse shape, these results illustrate a new way to tailor the continuum wave packet.


  • R. Ma
  • K. Motomura
  • K. L. Ishikawa
  • S. Mondal
  • H. Fukuzawa
  • A. Yamada
  • K. Ueda
  • K. Nagaya
  • S. Yase
  • Y. Mizoguchi
  • M. Yao
  • A. Rouze
  • A. Hundermark
  • M. J. J. Vrakking
  • M. Nagasono
  • K. Tono
  • T. Togashi
  • Y. Senba
  • H. Ohashi
  • M. Yabashi
  • T. Ishikawa
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Atom and Molecular Physics and Optics
Original languageEnglish
Article number164018
JournalJournal of Physics B: Atomic, Molecular and Optical Physics
Issue number16
Publication statusPublished - 2013
Publication categoryResearch