PT Unknown AU Hochstuhl, D TI Multiconfiguration methods for the numerical simulation of photoionization processes of many-electron atoms PY 2013 PU Christian-Albrechts-Universität zu Kiel WP https://macau.uni-kiel.de/receive/diss_mods_00011447 LA en DE Multiconfiguration methods; Multiconfigurational time-dependent Hartree-Fock; Time-dependent restricted active space configuration interaction; photoionization AB Numerical simulations present an indispensable way to the understanding of physical processes. In quantum mechanics, where the theoretical description is given in terms of the time-dependent Schrödinger equation (TDSE), the road is, however, difficult for any but the simplest systems. This is particularly true if one considers photoionization processes of atoms and mole\-cules, 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 composed of up to three particles. Despite this fundamental restriction, many physical effects can be essentially described by single- and two-electron models, among them high-order harmonic generation and non-sequential double-ionization of atoms and mole\-cules. 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 hence, no universally applicable method exists. The present work develops 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 previously. The MCTDHF method is already well established in the scientific community, but has been applied only rarely to photoionization processes so far. On the other hand, the TD-RASCI method is an original contribution, and is applied for the first time to solutions of the time-dependent Schrödinger equation. Further, through the invention of appropriate, grid-like single-particle basis sets, we adjust these general approaches to efficiently treat photoionization processes in many-electron atoms and molecules. After their thorough introduction, the MCTDHF and the TD-RASCI method are applied to several topics of photoionization physics. Among them is, first, the problem of calculating cross sections of atoms, for which we particularly consider helium, beryllium and neon. In most parts, this is accomplished for the first time in the framework of the developed methods. Next, we consider the two-photon double-ionization of helium, which has attracted considerable interest in recent years, and perform simulations with the MCTDHF method. We further apply the TD-RASCI method to study two-color pump-probe process in beryllium, the simulation of which requires an explicitly time-dependent treatment. We find that both methods are highly appropriate for accurately describing correlated single-ionization processes. Moreover, the TD-RASCI method is able to model relevant doubly-excited states, which are of central importance for a variety of physical processes. PI Kiel ER