Carrier dynamics on surfaces studied by two-photon photoemission

The lifetime of electronic excitations at surfaces play an important role for desorption processes induced by electronic transitions. Two-photon photoemission with time, energy, and angular resolution allows to study the scattering processes of excited electrons in detail. For electronic surface states various scattering channels can be identified, such as elastic vs. inelastic or intraband vs. interband. As examples the scattering of electrons in image-potential states by copper adatoms on Cu(0 0 1) and the decay and exciton formation in the dangling-bond surface conduction band at the Si(1 0 0) c(4 × 2) surface are presented. These studies of the carrier dynamics at surfaces imply that photochemical or electronic desorption processes are influenced by surface defects and are more efficient at semiconductor than on metal surfaces.