Abstract

The theory of multiphoton ionization for single electron atoms (or pseudo-one electron atoms such as sodium and cesium) is relatively well established from the standpoint of the knowing the relevant equations to be solved and interpreting the results. There is still some debate about the above threshold effects, but ionization rates have been calculated for high order processes, and the results agree well with experiment. This is not the case for multi-electron systems. In the limit of laser fields which are very strong compared to the Coulombic interactions, collective motion should be expected. At lower laser intensities, linear response theory or perturbation theory has proven to be adequate to model the absorption processes. In this regime, the inter-electronic forces are stronger than the photon-electron interaction so that the perturbation series is convergent. For the intermediate regime, say >10¹⁴W/cm², these two kinds of interactions should be treated on the same footing. A complete, exact, many body calculation for this problem is not possible so that several approximations have been considered. Independent particle models seem to be the most reasonable, with time dependent Hartree Fock being able to include most of the important physical effects, particularly as the number of electrons increases. The major approximation of this approach is that the state of the system is represented by a single, determinantal wave function for all time. This constraint limits the kinds of information which can be obtained from the calculations to averaged quantities. It has the merit, however, of including the effects of the time evolution of the electronic charge density on the absorption process. The redistribution of absorbed energy between all the system's electrons, and the rate at which this occurs is part and parcel of the calculations. This energy exchange is done self consistently so that an understanding of the total absorption dynamics is possible. This seems a minimum requirement in order to investigate any collective effects.

© 1986 Optical Society of America