Time-dependent variational approach for two-particle systems: photoionization of He (1s²)

An extension of the standard time-dependent Hartree-Fock (TDHF) approach to the problem of sequential multiple ionization of He by intense laser pulses is developed. The method is based on the TD Rayleigh-Ritz principle and employs a TD product wavefunction in coordinate space for He(1s, 1s'). While the method is comparable in effort to TDHF, it incorporates some additional two-particle correlation effects. Numerical results for high-frequency ionization of helium show deviations from TDHF results at moderate intensities, but, as in TDHF, simultaneous populations of He + and He 2+ according to binomial statistics are predicted in very strong fields. At optical laser frequencies and strong fields the results indicate a definite trend towards the sequential ionization mechanism suggested by experiment, but do not reproduce this effect fully.