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