Multiphoton processes in an intense laser field. II. Partial rates and angular distributions for ionization of atomic hydrogen at 532 nm

Abstract

We have performed nonperturbative (Floquet) calculations of partial and total rates for ionization of H(1s) by linearly polarized 532-nm light at intensities up to and above the threshold intensity (1.4\ifmmode\times\else\texttimes\fi{}${10}^{13}$ W/${\mathrm{cm}}^{2}$) at which the minimum number of photons required to ionize the atom increases from 6 to 7. Results are presented for partial seven- and eight-photon ionization rates, and for angular distributions in the seven-photon channel. (In accord with current conservation, the partial rates sum to the total rate.) We analyze, in some detail, the role of intermediate six-photon resonances between the 1s level and high Rydberg sublevels. Associated with each Rydberg manifold, specified by the principal quantum number (\ensuremath{\ge}9), are two resonance peaks in the ionization rate; one peak is very sharp, and corresponds to an avoided crossing of the quasienergy eigenvalues originating from the 1s level and some high Rydberg sublevel, while the other peak is broad (and smaller) and corresponds to a true crossing of the eigenvalues. The intermediate resonances significantly affect the angular distributions, and reveal the orbital angular momentum quantum number of the Rydberg sublevel involved in each resonance peak. Results are presented for the ionization yield and angular distribution produced by a realistic pulse, taking into account the temporal and spatial variation of the intensity, as well as ponderomotive scattering.