Polarization dependence of tunneling ionization of helium and neon by 120-fs pulses at 614 nm

We report on the polarization dependence of the ionization of helium and neon by 120-fs pulses at 614 nm. For linearly polarized pulses the data for ${\mathrm{He}}^{2+}$ and ${\mathrm{Ne}}^{2+}$ show enhancements over sequential tunneling. For circularly polarized pulses the data show no distinct enhancements. Dynamic resonances, which produce similar enhancements in the multiphoton regime, do not explain the data in the tunneling regime. Two direct-ionization models are considered: a shake-off model, and a semiclassical model for rescattering of an electron by the ion core. The exhibited polarization dependence of the ion yield enhancement is consistent with the semiclassical rescattering mechanism. Definitive isolation of a direct, nonsequential ionization mechanism in the optical tunneling regime requires continued scaling experiments on the laser wavelength, pulse width and ellipticity, and the target species.