Multiphoton detachment ofH−near the one-photon threshold: Exterior complex-scaling–generalized pseudospectral method for complex quasienergy resonances

We perform a nonperturbative study of the multiphoton above-threshold detachment of ${\mathrm{H}}^{\ensuremath{-}}$ in the presence of $1.640\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ and $1.908\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ laser fields by means of the non-Hermitian Floquet formalism. The laser parameters used are related to the recent experiments [L. Pr\ae{}stegaard, T. Andersen, and P. Balling (unpublished)] on the two-photon detachment of ${\mathrm{H}}^{\ensuremath{-}}$ near the one-photon threshold. The total and partial (above-threshold) detachment rates as well as the electron angular distributions are calculated for the laser intensities from ${10}^{9}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ to ${10}^{12} {\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$ It is found that at the weaker intensities (below ${10}^{11} {\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}),$ the perturbation theory provides a reasonable description of the two-photon detachment process and the detached electrons are largely in the d state. For higher intensity, however, the process becomes highly nonperturbative in nature. To perform the calculations, we have introduced an exterior complex-scaling\char21{}generalized pseudospectral (ECS-GPS) technique for the discretization and solution of the non-Hermitian Floquet Hamiltonian. The ECS-GPS procedure is accurate, simple to implement, and computationally more efficient than the basis-set expansion\char21{}variational methods for resonance-state calculations. It also provides a simpler procedure than the uniform complex-scaling method for the calculations of partial rates and electron angular distributions.