Strong Laser Fields
Welcome to the Strong Laser Fields Group at the Department of Molecular and Optical Physics, University of Freiburg, Germany.

Resonant and nonresonant multiphoton ionization of helium
Hanspeter Helm1 and Mark J. Dyer2
1Department of Molecular and Optical Physics, Albert-Ludwigs-Universität, Hermann-Herder-Strasse 3, D-79104  Freiburg, Germany
2Molecular Physics Laboratory, SRI International, Menlo Park, California 94025


We have investigated the multiphoton ionization of helium at wavelengths between 310 and 330 nm at intensities between 8 x 1013 and 5 x 1014 W/cm2 and at 630 nm at intensities of 1 x 1015 W/cm2. We characterize the ionization processes from photoelectron energy and angular distributions observed concurrently with photoion spectra. At the shorter wavelengths we find that resonant enhancement via the ac Stark shifted six-photon resonant states (1s3d and 1s3s) is a dominant ionization path as described previously by Perry, Szöke, and Kulander [Phys. Rev. Lett. 63, 1058 (1989)] and by Rudolph et al. [Phys. Rev. Lett. 66, 3241 (1991)]. At intensities above those required for resonant enhancement, and at wavelengths longer than those required for six-photon resonance, we observe that nonresonant seven-photon ionization dominates. This process gives rise to continuous distributions of low-energy electrons with characteristic angular distributions that peak near 0° and 60° relative to the laser polarization. At yet higher intensities, above the threshold where the nonresonant seven-photon channel closes, the dominant ionization path occurs via seven-photon resonant states with odd parity. This path gives rise to angular distributions characteristic of intermediate states with f character. ©1994 The American Physical Society

©1994 Phys. Rev. A 49, 2726-2733 (1994)

DOI: 10.1103/PhysRevA.49.2726
PACS: 33.80.Rv

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