Multielectron system in an ultrashort, intense laser field: A nonperturbative, time-dependent two-active-electron approach

We present a two-active-electron (TAE) approach for solving the time-dependent Schrödinger equation (TDSE) for the interaction of a multi-electron system with an ultrashort, intense, and linearly polarized laser pulse [Lagmago Kamta and Starace, Phys. Rev. Lett. 86, 5687 (2001)]. A technique for obtaining angular distributions for double ionization by such pulses is also described. The approach for solving the TDSE in the TAE approximation is full dimensional and accounts for correlations between the two electrons, as well as the polarization of the core. It is based on a configuration-interaction expansion of the time-dependent wave function in terms of one-electron atomic orbitals. Applying the method to the lithium negative ion $\left({\mathrm{Li}}^{-}\right),$ we display the time-dependent dynamics of the photodetachment process. For low intensities, our results for the detachment yield follow expectations from lowest-order perturbation theory and agree satisfactorily with $\mathcal{R}$-matrix calculations. Our results for angular distributions indicate that following multiphoton double ionization by an intense laser field, electrons are predominantly ejected along the laser polarization axis; however, a significant number are ejected perpendicularly to this axis. An angular momentum-based analysis of these angular distributions indicates that, in the dipole approximation and for an initial ${}^1{S}^e$ state interacting with a linearly polarized laser field, double ejection of both electrons along the direction perpendicular to the laser polarization axis can only occur following absorption of an even number of photons, whereas multiphoton absorption of an odd number of photons does not lead to double ejection at these angles.