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Vector momentum distributions of two electrons created in double ionization of Ar by 25 fs, 0.25 PW/cm(2) laser pulses at 795 nm have been measured using a "reaction microscope." At this intensity, where nonsequential ionization dominates, distinct correlation patterns are observed in the two-electron momentum distributions. A kinematical analysis of these spectra within the classical "recollision model" revealed an (e,2e)-like process and excitation with subsequent tunneling of the second electron as two different ionization mechanisms. This allows a qualitative separation of the two mechanisms demonstrating that excitation-tunneling is the dominant contribution to the total double ionization yield.

We present experimental and theoretical fully differential cross sections for single ionization of He by heavy-ion impact for electrons emitted into the scattering plane.Data were obtained for 2 MeV/amu C 6+ and 3.6 MeV/amu Au Q+ (Q = 24, 53) projectiles, corresponding to perturbations (projectile charge to velocity ratio) ranging from 0.7 to 4.4, a regime which is not accessible for electron-impact ionization. We observe a decreasing recoil peak intensity (relative to the binary peak) and at the same time an increasing peak in the forward direction with increasing perturbation. Large discrepancies between experiment and theory are found, which can at least partly be attributed to the use of hydrogenic wavefunctions.2

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