Theoretical Manifestation of the Broadening Effect on Photoelectron Angular Distributions

The broadening effect in photoelectron angular distributions (PADs) observed by Freeman et al. is studied theoretically. Using a nonperturbative scattering theory developed for multiphoton ionization with the inclusion of spontaneous emission, we calculate the PADs for above-threshold ionization (ATI) peaks. The numerical calculations from our theory reproduce the kinetic-energy dependence and the laser-intensity dependence of PADs of ATI peaks observed by Freeman et al., Phys. Rev. Lett. 57 (1986) 3156 and provide an evidence for the existance of the ponderomotive momentum of intense laser fields.