COMPUTATION OF STRONG-FIELD MULTIPHOTON PROCESSES IN POLYELECTRONIC ATOMS - STATE-SPECIFIC METHOD AND APPLICATIONS TO H AND LI-

The problem of integrating the time-dependent Schrödinger equation (TDSE) describing the interaction of a polyelectronic atom with a laser pulse is treated by expanding the time-dependent wave function (r,t) in terms of wave functions n,E computed for discrete, autoionizing, and scattering states separately. The TDSE is transformed into a system of coupled first-order differential equations with time-dependent coefficients, whose number (in the thousands), necessary for convergence to be reliable, depends mainly on the degree of the contribution of the continuous spectrum, as a function of the frequency and strength of the field. This approach allows the systematic incorporation of the significant electronic structure, electron correlation, and spectral characteristics of each N-electron system under investigation. Furthermore, since the free-electron function is computed numerically in the polarized core potential of the remaining (N-1)-electron atom, properties such as the angular distribution and partial above-threshold ionization (ATI) of the photoelectron are directly computable. We present results from the application of our methods to H and Li-. For the applications to H, which served as testing grounds for the method, the state-specific wave functions for discrete and continuum states were obtained numerically, for n and l up to 12 and 5, respectively, and for positive energies up to =34 eV with l up to 6. When comparisons with other time-independent and time-dependent results are possible, very good agreement is observed. On the other hand, our calculations do not confirm recent experimental results on absolute ionization rates for laser pulses of 248 nm. For Li-, our results on ATI for photon energy Latin small letter h with stroke=1.36 eV demonstrate the effects of initial-state electron correlation and of final-state field-induced coupling of open channels (the Li 1s22s 2S and 1s22p 2Po), as a function of field intensity. © 1994 The American Physical Society.