Ultrafast electron dynamics in Pb/Si(111) investigated by two-photon photoemission

Ultrafast electron dynamics in Pb/Si(111) quantum well structures have been investigated by femtosecond time-resolved two-photon photoemission spectroscopy. Three unoccupied quantum well states (or subbands) as well as an image potential state are identified and analyzed as a function of Pb thickness. Using 1.9 eV photon energy for optical excitation electron-hole pairs are formed in the Pb film and in the Si substrate. The hot electron population in the quantum well states exhibits a biexponential decay and both decay times increase when the energy difference to the Fermi level is reduced. The slower decay component with decay times of 130 and 900 fs is attributed to delayed filling of the states in the adlayer due to electronic relaxation in the Si(111) substrate. The faster decay (30-140 fs) is caused by electron-electron scattering in the Pb film. At a first glance, the extracted decay times increase toward the Fermi level as predicted by the Fermi-liquid theory. However, by investigation of the quantized electronic structure more detailed insight into scattering among defined subbands has been gained. The rise of the excited electron population in the lowest unoccupied quantum well state is delayed by 70 fs which occurs concomitantly with a decay in the second lowest state. We describe this simultaneous decay and rise in the two adjacent subbands by coupled rate equations and attribute it to intersubband scattering. Thereby, we demonstrate the importance of intersubband decay in low-dimensional metallic systems in addition to intraband scattering processes considered in Fermi-liquid theory and derived descriptions.