Above-threshold multiphoton detachment ofH−by two-color laser fields: Angular distributions and partial rates

We present a general nonperturbative formalism and an efficient and accurate numerical technique for the study of the angular distributions and partial widths for multiphoton above-threshold detachment in two-color fields. The procedure is based on an extension of our recent paper [D. A. Telnov and S.-I Chu, Phys. Rev. A 50, 4099 (1994)] for one-color detachment, and the many-mode Floquet theory [T. S. Ho, S.-I Chu, and J. V. Tietz, Chem. Phys. Lett. 96, 464 (1983)]. The generalization of this procedure is performed for both cases of commensurable and incommensurable frequencies of the two-color fields. The procedure consists of the following elements: (i) Determination of the resonance wave function and complex quasienergy by means of the non-Hermitian Floquet Hamiltonian formalism. The Floquet Hamiltonian is discretized by the complex-scaling generalized pseudospectral technique recently developed [J. Wang, S.-I Chu, and C. Laughlin, Phys. Rev. A 50, 3208 (1994)]. (ii) Calculation of the angular distribution and partial widths based on an exact differential formula and a procedure for the rotation of the resonance wave function back to the real axis. The method is applied to a nonperturbative study of multiphoton above-threshold detachment of ${\mathrm{H}}^{\mathrm{-}}$ by 10.6-μm radiation and its third harmonic (the commensurable case). The results show strong dependence on the relative phase δ between the fundamental frequency field and its harmonic. For the intensities used in calculations (${10}^{10}$ W/${\mathrm{cm}}^2$ for the fundamental frequency, ${10}^8$ and ${10}^9$ W/${\mathrm{cm}}^2$ for the harmonic), the total rate has its maximum at δ=0 and minimum at δ=π. However, this tendency, though valid for the first several above-threshold peaks in the energy spectrum, is reversed for the higher-energy peaks. The energy spectrum for δ=π is broader, and the peak heights decrease more slowly compared to the case of δ=0. The strong phase dependence is also manifested in the angular distributions of the ejected electrons.