PHYSICAL REVIEW A VOLUME 49, NUMBER 4 APRIL 1994 Resonant and nonresonant multiphoton ionization of helium Hanspeter Helm and Mark J. Dyer Molecular Physics Laboratory, SRI International, Men lo Park, California 94025 (Received 7 May 1993) We have investigated the multiphoton ionization of helium at wavelengths between 310 and 330 nm at intensities between 8X10' and 5X10' W/cm and at 630 nm at intensities of 1X10' W/cm . We characterize the ionization processes from photoelectron energy and angular distributions observed con- currently 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, Szoke, 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 dom- inant ionization path occurs via seven-photon resonant states with odd parity. This path gives rise to an- gu1ar distributions characteristic of intermediate states with f character. PACS number(s): 33. 80. Rv INTRODUCTION Atomic helium is an attractive system for studying the response of atoms to irradiation by intense laser fields [1, 2]. One reason is that the relative simplicity of helium permits theory to be carried out to a high level. To the experimentalist helium poses an interesting challenge ow- ing to its high ionization potential and the associated sen- sitivity of the experiment to impurities. While several re- cent studies on the mass-selected ion yield from multi- photon ionization of helium have appeared in the litera- ture [3, 4], we are aware of only a single experiment describing photoelectron spectroscopy from strong-field multiphoton ionization in the near uv. Perry, Szoke, and Kulander [1] reported investigations of photoelectron en- ergy distributions that arise from (6+1)-photon ioniza- tion of helium via the ( ls3d) state and (5+1)-photon ion- ization via the ( is2p) state. In these experiments, reso- nance is achieved when, at a critical intensity in the laser pulse, the intermediate state is ac Stark shifted into reso- nance with the 1V-photon dressed ground state. When a laser of short time duration is used, the ionization of the intermediate gives rise to photoelectrons with energies characteristic of the ionization potential of the intermedi- ate [5], thereby enabling assignment of the photoelectron peaks. The ac Stark shift observed in these experiments has been confirmed by quantitative theory [1, 2], leaving us with the result that even at intensities as high as 4X 10' W/crn the dynamic response of helium is essen- tially that of a one-electron system, at least in the wave- length range from 280 to 297 nrn. Here we report studies of photoelectron energy and an- gular distributions from helium obtained in the range from 310 to 330 nm. Our studies essentially confirm the earlier results [1, 2], but they also show that, upon raising the intensity beyond that required for resonance, non- resonant ionization eSciently competes with the resonant channel. We also show that at intensities above which the nonresonant channel closes due to the rise of the ion- ization potential with intensity, resonant excitation reoccurs for states with 1 higher angular momentum. The situation in the near uv is quite different from what is observed at visible wavelengths where tunnel ionization dominates. EXPERIMENT A charged-particle-imaging spectrometer [6] is em- ployed to observe the energy and angular distribution of photoelectrons. In this device, photoelectrons expanding from the focal volume are projected onto a phosphor screen by a dc electric field, as shown schematically in Fig. 1. The spatial image of photoelectrons is recorded for typically 10 — 100 ionization events per laser shot us- ing a video camera. Several thousand video frames are digitized and added to produce images such as those shown on the screen to the right in Fig. 1. In such a pro- jection, photoelectrons of constant energy fall inside a circular pattern, the diameter of which is related to the electron velocity. The filling pattern inside the circle is related to the angular distribution of photoelectrons, which controls the density of electrons on the surface of the expanding constant-energy sphere. In the projection method, the sphere diameter appears with enhanced in- tensity owing to the singularity in the Jacobian that de- scribes the transformation from the spherical to the pla- nar coordinate system. This enhancement aids in the identification. All images shown here are recorded with linear laser polarization, oriented parallel to the screen and along the vertical axis of all images shown in this pa- per. Recording images (under otherwise identical condi- tions) with the laser polarization perpendicular to the screen reveals the cylindrical symmetry of the distribu- 1050-2947/94/49(4)/2726(8)/$06. 00 49 2726 1994 The American Physical Society