Date of this Version
Above-threshold detachment (ATD) rates for H1 and F1 ions in the high-energy plateau region are calculated as functions of photon number and laser intensity by solving the time-dependent Schrödinger equation within the Sturmian-Floquet approach. Pronounced enhancements of the ATD rates are found (up to an order of magnitude) as the laser-field intensity passes across ponderomotive-potential-induced channel closings. We confirm the zero-range potential model results of Borca et al. [Phys. Rev. Lett. 88, 193001 (2002)] for negative ions whose initial states have s symmetry, and we investigate here the case of initial states having p symmetry. Depending on the symmetry of the initial state, the enhancement is found to be most pronounced for even- or odd-channel closures, which is consistent with threshold laws applicable at the closing of particular multiphoton channels. Variations of ATD electron angular distributions as functions of laser intensity near channel closings are also investigated and found to be sensitive to initial-state symmetry.