New Journal of Physics (Jan 2013)

Near-threshold photoelectron angular distributions from two-photon resonant photoionization of He

  • P O'Keeffe,
  • A Mihelič,
  • P Bolognesi,
  • M Žitnik,
  • A Moise,
  • R Richter,
  • L Avaldi

DOI
https://doi.org/10.1088/1367-2630/15/1/013023
Journal volume & issue
Vol. 15, no. 1
p. 013023

Abstract

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Two-photon resonant photoionization of helium is investigated both experimentally and theoretically. Ground state helium atoms are excited to the 1s4p, 1s5p and 1s6p ^1 P states by synchrotron radiation and ionized by a synchronized infrared pulsed picosecond laser. The photoelectron angular distributions of the emitted electrons are measured using a velocity map imaging (VMI) spectrometer. The measured asymmetry parameters of the angular distribution allow the phase differences and the ratios of the dipole matrix elements of the 1s ϵ s and 1s ϵ d channels to be determined. The experimental results agree with the calculated values obtained in a configuration–interaction calculation with a Coulomb–Sturmian basis set. The effects of the radiative decay of the intermediate state and the static electric field of the VMI spectrometer on the measurements are discussed.