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 Schrodinger equation (TD SE) describing the interaction of a polyelectronic atom with a laser p ulse is treated by expanding the time-dependent wave function psi(r,t) in terms of wave functions Phi(n,E) computed for discrete, autoionizi ng, and scattering states separately. The TDSE is transformed into a s ystem of coupled first-order differential equations with time-dependen t coefficients, whose number (in the thousands), necessary for converg ence to be reliable, depends mainly on the degree of the contribution of the continuous spectrum, as a function of the frequency and strengt h of the field. This approach allows the systematic incorporation of t he significant electronic structure, electron correlation, and spectra l characteristics of each N-electron system under investigation. Furth ermore, since the free-electron function is computed numerically in th e polarized core potential of the remaining (N-1)-electron atom, prope rties such as the angular distribution and partial above-threshold ion ization (ATI) of the photoelectron are directly computable. We present results from the application of our methods to H and Li-. For the app lications to H, which served as testing grounds for the method, the st ate-specific wave functions for discrete and continuum states were obt ained numerically, for n and l up to 12 and 5, respectively, and for p ositive energies up to epsilon = 34 eV with l up to 6. When comparison s 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 rate s for laser pulses of 248 nm. For Li-, our results on ATI for photon e nergy h ($) over bar omega=1.36 eV demonstrate the effects of initial- state electron correlation and of final-state field-induced coupling o f open channels (the Li 1s(2)2s(2)S and 1s(2)2p(2)P degrees), as a fun ction of field intensity.