Multiphoton autoionization under strong laser radiation: Three-photon autoionization of strontium as a test case

A theory of two- and three-photon autoionization is presented for intermediate resonant and non-resonant cases, stressing the laser-intensity effect on the configuration mixing of the final state. Sample calculations are performed for the $5p6s{}_{}{}^1{}_{}{}^{}P$ autoionizing state. The involvement of the $5s5d$ intermediate state includes the effects of ac Stark shifts on the ionization rate, the line shape, and the photoelectron angular distributions.