POLARIZATION DEPENDENCE OF TUNNELING IONIZATION OF HELIUM AND NEON BY120-FS PULSES AT 614 NM

We report on the polarization dependence of the ionization of helium a nd neon by 120-fs pulses at 614 nm. For linearly polarized pulses the data for He-2+ and Ne2+ show enhancements over sequential tunneling. F or circularly polarized pulses the data show no distinct enhancements. Dynamic resonances, which produce similar enhancements in the multiph oton regime, do not explain the data in the tunneling regime. Two dire ct-ionization models are considered: a shake-off model, and a semiclas sical model for rescattering of an electron by the ion core. The exhib ited polarization dependence of the ion yield enhancement is consisten t with the semiclassical rescattering mechanism. Definitive isolation of a direct, nonsequential ionization mechanism in the optical tunneli ng regime requires continued scaling experiments on the laser waveleng th, pulse width and ellipticity, and the target species.