Relativistic MeV Photoelectrons from the Single Atom Response of Xenon to a 10$^{19}$ W/cm$^{2}$ Laser Field

We present experimental photoelectron measurements from the single atom photoionization of Xe exposed to field intensities up to 1.2x10$^{19}$ W/cm$^{2}$. An ultra-strong laser field was used to ionize Xe and the resulting high energy electrons as a function of intensity, energy and angle were measured with a dynamic range of four orders of magnitude. The measurements are compared to a 3D, relativistic, semi-classical, single electron model of ionization [1]. The essential photoelectron spectrum features above 0.5 MeV, including the high energy cutoff, are in reasonable agreement with a semi-classical, relativistic 3D model of ionization. The observed energy spectrum and angular distributions at 60 keV is lower than the calculated result by an order of magnitude indicating existence of multi-electron processes which are not included in the model. This work is supported by the National Science Foundation (Grant {\#}: 0757953).\\[4pt] [1] I. Ghebregziabher and B.C. Walker, PRA 76, 023415 (2007).