Strong-field ionization of few-electron systems with MCTDHF

Abstract

This thesis describes the Multi-Configuration Time-Dependent Hartree-Fock (MCTDHF) method for solving the multi-electron time-dependent Schrödinger equation, and its application to strong laser physics.<br />In part one we outline the theory and practical implementation of the MCTDHF method. We define the term "correlation" as anything which goes beyond Hartree-Fock, and point out an efficient way of dealing with the numerical challenges correlation creates. This results in the MCTDHF ansatz, where the multi-electron wave function is expanded in a sum of Slater determinants, with time-dependent single-electron orbitals and coefficients. MCTDHF allows to systematically improve treatment of correlation, while reducing computational effort substantially. Our parallelized MCTDHF implementation makes calculations of up to 6 electrons feasible within a tolerable time range on the order of days. In principle, any Hamiltonian and any geometry can be fitted in our program framework. Originally, the method was developed for and applied to strong-field problems, such as ionization and high-harmonic generation (HHG), which are the topics of this work.<br />In part two, we investigate multi-electron effects on the strong-field ionization of molecules. We perform with MCTDHF the first calculation of this process both in 3D and with correlated electrons. Contrary to some experimental observations, we find that ionization increases with the size of the molecule in 3D. Further we show that simplified 1D models are misleading already at a qualitative level.<br />The third part deals with multi-electron dynamics in high-harmonic generation of molecules. Until recently, HHG was almost uniformly interpreted in terms of a single-active electron model, most notably within the strong-field approximation (SFA).<br />First we investigate the dependence of the re-colliding electron wave packet on electron correlation effects.<br />This is followed by an extensive in-depth analysis of HHG in molecules.<br />We calculate high harmonic spectra with MCTDHF for diatomic molecules with 2 and 4 active electrons, and compare the results with several simplifying models, all of which leave out multi-electron dynamics. In fact, none of them can qualitatively reproduce the multi-electron results.<br />By factorization of the total wave function into the ionic core and a single-electron orbital, we demonstrate explicitly that polarization dynamics of the multi-electronic core has to be taken into account. These findings dismiss any hope of single-electron based description of HHG in molecules. In the last and fourth part, we address a question from laser interaction with solids. To explain optical breakdown of dielectrics at very short pulse-lengths, the so-called ``forest fire'' mechanism was proposed, where a charged hole enhances laser ionization at neighboring atoms. Using a two-electron model of this process, we find however no evidence for hole-assisted laser ionization.