Multiphoton detachment rates of H- for weak and strong fields - art. no. 013410

The time-independent Schrodinger problem for the H- plus ac field system ha s been solved from first principles via the nonperturbative many-electron, many-photon theory (MEMPT) for a wide range of values of frequency omega an d of intensity I of linearly polarized light. The calculations obtained the multiphoton electron detachment rates (MPEDRs) as the imaginary part of a complex eigenvalue and were done for combinations of values of omega and of I defining regimes of ''weak" and of "strong'' fields. Most of the results cover the cases of two-, three-,..., seven-photon electron detachment, stu died as a function of frequency and of intensity. However, special cases, s uch as the one of I = 2 x 10(11) W/cm(2) for the CO2 frequency of 0.117 eV, represent detachment processes into various symmetries requiring the absor ption of at least 25 photons. The MEMPT results were obtained without any e mpirical adjustment of energies or of basis sets. The dressed-atom resonanc e wave function consisted of optimized function spaces for the initial and final states, including the lowest S-1, P-1(o), and D-1 doubly excited stat es (DES). The initial state was represented by a ten-term numerical multico nfigurational Hartree-Fock wave function whose energy, -0.5275 a.u., is ver y close to the exact one, -0.5277 a.u., and which accounts self-consistentl y for electron correlation as well as for the proper magnitude of the 1s or bital at large values of r. The H- DES wave functions were correlated, yiel ding accurate energies. However, their presence does not affect the results at all. The results converged well when 15 photon blocks were used. In spi te of the large number of absorbed photons required in cases such as the CO 2 frequency, the calculations converged well, within the numerical accuracy of the algorithms, by using free-electron angular momenta with 1 up to 7. The systematic quantitative study of the dependence of the MPEDRs on omega and I has led to conclusions as to the behavior at thresholds and as to the limits of validity of the predictions of the lowest-order perturbation the ory. An interesting result is the appearance of intensity-dependent structu res in the two-, four-, and six-photon detachment rates, which is caused by the interference of the S-1 and D-1 channels. For a number of (I,omega) pa irs, comparison is possible with published results obtained by earlier larg e-scale calculations which either started from first principles or used par ametrized one-electron models. Overall, there is good agreement. We conclud e that the current level of theoretical knowledge of the H-MPEDR spectrum i s very satisfactory for a large set of experimentally possible laser parame ters.