Unoccupied electronic structure and relaxation dynamics of Pb/Si(111)

The unoccupied electronic structure of epitaxial Pb films on Si(1 1 1) is analyzed by angle-resolved tw photoemission in the (Gamma) over bar --> (M) over bar direction close to the Brillouin zone center. The experimental results are compared to density functional theory calculations and we focus on the nature of the interaction of the 6p(z) states with the Si substrate. The experimentally obtained dispersion E(k(parallel to)) of the unoccupied quantum well states is weaker than expected for freestanding films, in good agreement with their occupied counterparts. Following E(k(parallel to)) of quantum well states as a function of momentum at different energies, which are degenerate and non-degenerate with the Si conduction band, we observe no influence of the Si bulk band and conclude a vanishing direct interaction of the Pb 6p(z) states with the Si band. However, the momentum range at which mixing of 6p(z) and 6p(x,y) derived subbands is found to occur in the presence of the Si substrate is closer to (Gamma) over bar than in the corresponding freestanding film, which indicates a substrate-mediated enhancement of the mixing of these states. Additional femtosecond time-resolved measurements show a constant relaxation time of hot electrons in unoccupied quantum well states as a function of parallel electron momentum which supports our conclusion of a p(x,y) mediated interaction of the p(z) states with the Si conduction band. (C) 2014 Published by Elsevier B.V.