Above-threshold multiphoton detachment of H- by two-color laser fields: Angular distributions and partial rates.

Abstract

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 ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ by 10.6-\ensuremath{\mu}m radiation and its third harmonic (the commensurable case). The results show strong dependence on the relative phase \ensuremath{\delta} 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 \ensuremath{\delta}=0 and minimum at \ensuremath{\delta}=\ensuremath{\pi}. 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 \ensuremath{\delta}=\ensuremath{\pi} is broader, and the peak heights decrease more slowly compared to the case of \ensuremath{\delta}=0. The strong phase dependence is also manifested in the angular distributions of the ejected electrons.