Abstract
A simple model is introduced to investigate the temporal evolution of the density of electrons which are excited from a metal substrate to a localized state at the surface by a laser pulse. The electron system consists of two subbands in the substrate and a localized state at the surface, and the localized state is in resonance with the upper subband. The obtained expression for the electron density in the localized state indicates that the excitation process can be divided into two, i.e., direct and indirect processes. In the latter, the transition probability reaches a maximum at a certain photon energy (absorption peak energy /) because of the momentum conservation. This process contributes substantially to the electron density and hence strongly affects its temporal evolution at the incident photon energy around the absorption peak energy.
