Ultrafast hole dynamics in two-photon photoemission from metal surfaces

We investigate the effects of photoexcited-hole decay on time-resolved two-photon photoemission spectra of metal surfaces by non-equilibrium Green function method. We show that it is necessary for calculating the photoelectron intensity to introduce a Green function depending on the observation time of the photoelectron. The photoexcited-hole decay causes the loss of coherence of the system and hence accounts for acceleration of the decay of the photoexcited-electron density. Then the effective lifetimes estimated from the photoelectron intensity as a function of the pump–probe delay time can be the lifetimes of the photoexcited ‘(non-excitonic) electron–hole pairs’ even when direct interactions between photoexcited electrons and holes are negligible while it is usually considered that the effective lifetimes correspond to the electron lifetimes.