Ultrafast electron dynamics in Pb/Si(111) quantum well structures has been investigated by femtosecond time-resolved two-photon photoemission spectroscopy. Four 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 bi-exponential decay and both decay times increase when the energy difference to the Fermi level is reduced. The slower decay component with a decay time of 130 fs (900 fs) for the second (first) unoccupied quantum well state 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 towards 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 inter-subband scattering. Thereby, we demonstrate the importance of inter- subband decay in low-dimensional metallic systems in addition to intra-band scattering processes considered in Fermi-liquid theory and derived descriptions.
The original publication is available at www.springerlink.com by link DOI: 10.1103/PhysRevB.78.035437