Band structure dependence of hot-electron lifetimes in a Pb/Cu(111) quantum-well system

The band structure dependence of the inelastic lifetime of electrons is investigated in a Pb quantum-well system on a Cu(111) substrate with femtosecond time-and angle-resolved two-photon photoemission. For a single monolayer of Pb on Cu(111), we find an unoccupied quantum-well state with a free-electron-like parabolic dispersion around the (Gamma) over bar point and with negative dispersion for finite momentum. Our investigation of this state is a momentum-dependent study of hot-electron lifetimes in a quantum-well system. We demonstrate the importance of intrasubband-scattering processes for the decay of hot electrons at finite momentum. Furthermore, we find that the competition between intersubband- and intrasubband-scattering processes directly induces a momentum anisotropy in the hot-electron lifetimes. This momentum anisotropy is strongly dependent on the specific electronic band dispersion. We compare our findings with previous investigations of ultrafast electron dynamics in model-like surface-state systems and with ultrafast electron dynamics in Pb full-bulk material. Our findings suggest that the peculiar electronic structure of quantum-well systems can be used to tune ultrafast dynamical properties in metals.