Fast initial decay of molecular excitations at insulator surfaces.

We investigate the initial decay of electron-hole excitations in a molecular layer after adsorption on an insulator substrate surface. This is done within ab initio many-body perturbation theory by calculating the time propagation of excited two-particle states. For a CO monolayer adsorbed on the MgO(001) surface we find very fast decay processes with lifetimes that are about 5 times shorter than the transfer of single charge carriers.

Image states and excitons at insulator surfaces with negative electron affinity.

We discuss electronic excitation processes at two ionic insulator surfaces, LiF(001)-(1x1) and MgO(001)-(1x1), within ab initio many-body perturbation theory. Because of the negative electron affinity of the surfaces, the lowest unoccupied electronic states are image states located in the vacuum outside the surface. Excitations of electrons from the surface layer into these image states are much lower in energy than the bulk excitons.

Localization of optically excited states by self-trapping.

We discuss the localization of the surface exciton at the Si(111)- (2x1) surface due to self-trapping, which leads to a characteristic temperature-dependent linewidth of the optical response and to a significant Stokes shift of the luminescence. Self-trapping results in this case from a structural relaxation in the excited state, caused by the interplay between electronic and geometric degrees of freedom. The most significant contribution to this effect comes from one single geometric deformation mode which is driven by the internal electronic charge transfer in the self-trapped exciton.