INTENSITY-DEPENDENT SCATTERING RINGS IN HIGH-ORDER ABOVE-THRESHOLD IONIZATION

Angular distributions of high energy photoelectrons from Xe and Kr, ex cited by a 50 ps, 1.05 mum laser, are presented. In Xe, strong, narrow rings approximately 45-degrees off the polarization axis appear in a limited energy range centered around 9U(p), where U(p) is the ponderom otive energy. This effect is much weaker in Kr. Single active electron calculations agree well with these observations. We conclude that the rings result from single-electron ionization dynamics, most likely in volving rescattering from the ion core of the tunneling component of t he continuum wave function.