Multiphoton ionization of atoms: Effect of the final-state Coulomb potential on the ionization rate

An approximation scheme is proposed to incorporate the influence of the Coulomb potential on the photoelectron in a nonperturbative treatment of multiphoton ionization rates. The method involves an approximation to an exact rate expression; when the latter is expanded in powers of the binding potential, the zeroth-order term is the commonly used Keldysh rate. The approach is applied to hydrogenlike atoms. Photoelectron angular distributions are calculated; these compare favorably with experimental measurements and with results of some other theoretical approaches.