Dephasing in Two-Photon Photoemission from Metal Surfaces : Effects of Hole Scattering

Microscopic mechanisms of energy relaxation and dephasing in two-photon photoemission (2PPE) from metal surfaces are investigated by nonequilibrium Green function method. It is shown that the dephasing time corresponds to the lifetime of a hole excited in an occupied state by a pump photon. The hole lifetime also affects the energy relaxation time which is usually considered to be the intrinsic electron lifetime of an unoccupied state. The density matrix method can reproduce the results by the Green function method well, however, fails to estimate the effects of the hole scattering properly. Based on the obtained results, a new physical interpretation of the energy relaxation time and the dephasing time of metals is proposed.