We present energy-and angle-resolved photoionization from Xe in an ultrastrong laser field at 10 19 W/cm 2 . The observed yields are consistent with the tunneling ionization of Xe 9+ to Xe 24+ . However, energy and angle-resolved photoelectron spectra show differences for electrons whose final energies are above or below 0.5 MeV, which is approximately the ponderomotive energy at these intensities. Above 0.5 MeV, the observed photoelectron cutoff energy (between 1 and 1.35 MeV), photoelectron energy spectra, and the angle-resolved photoelectron azimuthal distributions agree with a model using tunneling ionization, multiple charge states, a classical relativistic continuum, and nonparaxial three-dimensional (3D) focused laser field. Below 0.5 MeV the yields and angular distributions observed indicate dynamics not included within a classical, single electron model of the interaction.
The ionization of Ne and Kr at 400 nm has been measured from 10 13 to 10 17 W/cm 2 for charge states up to Kr 8+ . Nonsequential ionization is seen in the higher charge states Ne 3+ , Ne 4+ , Ne 5+ , Kr 5+ , Kr 6+ , Kr 7+ , and Kr 8+ when the atomic response to the increasing external field strength moves from multiphoton to tunneling. A similar development in nonsequential ionization is seen in a semiclassical model and an empirical model of rescattering. The models show that nonsequential ionization not apparent in lower charge states can become prominent for higher charge states. It is possible to generalize these results and predict optimal wavelengths for multielectron excitation via nonsequential ionization. The findings indicate that ultraviolet to vacuum ultraviolet wavelengths can give the largest recollision for higher charge states.
We present the ellipticity dependence of the ultrafast photoionization for Cn+ fragments from methane. The study extends from the strong field (C+, C2+) at 1014 W cm−2 to the ultrastrong field (C5+) at 1018 W cm−2. The measurements show that C+ and C2+ ionization have limited sensitivity to the field polarization. As the laser intensity and corresponding degree of ionization increase (C4+, C5+), the dependence on the field polarization increases. Comparison to a semi-classical field ionization model shows that the ellipticity dependence of the relative ion yield for higher charge states comes from the field dependence of tunnelling ionization rather than nonsequential ionization due to rescattering. A movement from a molecule-like response to an atom-like response with the increase in intensity is observed.
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