Electron Shell Ionization with Classical, Relativistic Scattering by Ultrastrong Fields: the Single Atom Response

We investigate the forward scattering of ionization from neon, argon, and xenon in ultrahigh intensities of 2 $\times$ 10$^{\mathrm{19}}$ W/cm$^{\mathrm{2}}$. Comparisons between the gases reveal the energy of the outgoing photoelectron determines its momentum, which can be scattered as far forward as 45 degrees from the laser wavevector for energies greater than 1 MeV. The shell structure in the atom manifests itself as modulations in the photoelectron yield and the width of the angular distributions. We arrive at an agreeable comparison with theory using an independent electron model for the atom, dipole approximation for the bound state interaction and a relativistic, three-dimensional, classical radiation field including the laser magnetic field for continuum dynamics. The studies presented here are at the highest intensity yet observed in single atom studies. This work is supported by the Army Research Office under Award No. W911NF-09-1-0390 and National Science Foundation under Award No. 0757953. MFD acknowledges support from the DOE-EPSCoR grant DE-FG02-11ER46816.