THE HELIUM ATOM IN STRONG AND SHORT LASER PULSES: MULTIELECTRON EFFECTS

摘要

Two-electron ejection at low frequencies receives a const ant attention since 15 years from both theoretical and experimental sides. We note in particular the recent measurements of Weber et al (2000) and Moshammer et al (2000), which provide informations on the total ion momentum distribution in the non-sequential regime. Numerically, simulating double electron ionization at 800 nm is an enormous challenge. The main difficulty is that the full resolution of this problem requires the non-perturbative solution of the time-dependent Schrodinger equation (TDSE) together with an accurate representation of electron correlations. These latter requirements lead to very demanding calculations. Much has been learned in the single active electron model (SAE) but very little is known beyond the approximations sustaining the time-dependent Hartree-Fock (TDHF) approach (see Kulander, 1987, 1988, Horbatsch et al, 1992, Maragakis and Lambropoulos, 1997 and Hasbani et al, 2000). Nevertheless, progress have been realized. In particular the resolution of the TDSE with approaches based on finite element developments have been shown to be particularly efficient in the XUV domain. For high frequency fields and femtosecond regime, theoretical investigations are stimulated by the perspectives offered by the harmonic generation and free electron laser sources (FEL). In this context the spectral method has been shown to be particularly efficient. It consists in developing the time-dependent solution of the Schrodinger equation on a truncated basis of antisymmetrised products of orbital func- tions(see Zhang and Lambropoulos, 1995). This type of method includes explicitly the characteristics of the pulse(e.g.the temporal dependence) and the atomic structure (bound and autoionizing states, continua..). Also, it is worth noticing that the various informations on the fintal product of the laser-atom interaction (bound-state populations, ionizasion yield and angular distribution of ejected electrons, branching ratio in electron spectra..) can be easily extracted from the wavefunction at the end of the pulse. Another approach consists in describing the radial wavefunction on a grid. This method is capable of modelling intense-field two-electron effects such as double photoionization (see Dundas et al, 1999) or autoionization effects (see Parker et al, 2000). Due to memory limitation on available computers, rather severe space-time limitation constrains exist. Nevertheless the grid method has been adapted for parallel processing (see Smyth et al, 1998) and it takes advantage of the rapid development of massively parallel machines. Details and references on the various approaches are reviewed in Lambropoulos et al, 1998. Most of the more recent developments are presented in the proceedings of the SILAP2000 Nato Advanced Research Workshop.