PhD Paul Tiwald

Local Electronic Excitations in Extended Systems: A Quantum-Chemistry Approach


Real solids and surfaces are not "perfect". Crystals inevitably contain various defects and surfaces are subject to interactions with ambient particles leading, for example, to oxidation and adsorption. Since such effects are present in everyday devices and applications a deep understanding of the underlying physics is of great importance. In this thesis we study the properties of two very localized imperfections: the F-type color center in alkali-halide crystals and the charge transfer during scattering of an ion from an insulator surface. Both effects have been studied for a long time but a detailed theoretical understanding on the ab-initio level seems to be missing.

This thesis provides state-of-theart ab-initio calculations and addresses open questions. In particular, we present an ab-initio study of the physics underlying the so-called Mollwo-Ivey relation. This relation connects the F-center absorption energies with the crystal lattice constants and has not been fully understood so far. Second, we present the first ab-initio results on the charge-transfer probability during scattering of a proton from a lithium-fluoride surface. This study is based on a non-adiabatic molecular dynamics approach that provides microscopic insight into the charge-transfer process. Both the light absorption by the color center and the charge transfer represent local electronic excitations: the F-type color center consists of an electron strongly localized at an anionic vacancy and the electron transferred is strongly localized in close vicinity of the proton. This localization allows for application of the so-called embedded cluster approach in which the extended system is approximated by an embedded finite-sized active cluster. To study the properties of the active clusters we apply high-level quantum chemistry methods solving the electronic Schrödinger equation.