Time-dependent multiconfiguration methods for the numerical simulation of photoionization processes of many-electron atoms

Abstract

Numerical simulations present an indispensable way to the understanding of complex physical processes. In quantum mechanics where the theoretical description is given in terms of the time-dependent Schrodinger equation the road is, however, difficult for any but the simplest systems. This is particularly true if one considers photoionization processes of atoms and molecules which, at the same time, require an accurate description of bound and continuum states and, therefore, an extensive region of space to be sampled during the calculation. As a consequence, direct simulations of photoionization processes are currently only feasible for systems containing up to three electrons. Despite this fundamental restriction, many physical effects can be essentially described by single- and two-electron models, among them are high-order harmonic generation and non-sequential double-ionization of atoms and molecules. A plethora of numerical investigations have been performed on atomic and molecular hydrogen and helium in the last two decades, and these have had a strong impact on the current understanding of photoionization. On the other hand, there are processes which are characterized by the interplay of a larger number of electrons, such as tunnel ionization, the Auger effect and, to give a more recent example, the temporal delay between the photo-emission of electrons from different shells of neon and krypton. The many-electron character of these effects complicates the accurate, time-resolved simulation and, so far, no universally applicable method exists. This review presents two theoretical methods which are promising candidates for closing this gap–the multiconfigurational time-dependent Hartree-Fock (MCTDHF) method and the time-dependent restricted active space configuration interaction (TD-RASCI) method. Both represent the wavefunction in a linear subspace of the many-body Hilbert space and follow particular strategies to avoid the exponential problem. This makes it possible to treat a much larger number of electrons than with the direct techniques mentioned above.