Resonant and nonresonant multiphoton ionization of helium

We have investigated the multiphoton ionization of helium at wavelengths between 310 and 330 nm at intensities between 8×${10}^{13}$ and 5×${10}^{14}$ W/${\mathrm{cm}}^2$ and at 630 nm at intensities of 1×${10}^{15}$ W/${\mathrm{cm}}^2$. 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.