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. This is due to a strong coupling of the molecular excitations to charge-transfer-exciton states between the adlayer and the substrate.