Multiphoton detachment of H-. II. Intensity-dependent photodetachment rates and threshold behavior-complex-scaling generalized pseudospectral method

We extend our previous perturbative study of the multiphoton detachment of H[sup [minus]] [Phys. Rev. A 48, 4654 (1993)] to stronger fields by considering the intensity-dependent photodetachment rates and threshold behavior. An accurate one-electron model potential, which reproduces exactly the known H[sup [minus]] binding energy and the low-energy [ital e]-H(1[ital s]) elastic-scattering phase shifts, is employed. A computational technique, the complex-scaling generalized pseudospectral method, is developed for accurate and efficient treatment of the time-independent non-Hermitian Floquet Hamiltonian [ital [cflx H]][sub [ital F]]. The method is simple to implement, does not require the computation of potential matrix elements, and is computationally more efficient than the traditional basis-set-expansion--variational method. We present detailed nonperturbative results of the intensity- and frequency-dependent complex quasienergies ([ital E][sub [ital R]],[minus][Gamma]/2), the complex eigenvalues of [ital [cflx H]][sub [ital F]], providing directly the ac Stark shifts and multiphoton detachment rates of H[sup [minus]]. The laser intensity considered ranges from 1 to 40 GW/cm[sup 2] and the laser frequency covers 0.20--0.42 eV (in the c.m. frame). Finally we perform a simulation of intensity-averaged multiphoton detachment rates by considering the experimental conditions of the laser and H[sup [minus]] beams. The results (without any free parameters) are in good agreement with experimental data, both in absolute magnitude and in the threshold behavior.