Unbesetzte elektronische Struktur und Relaxationsdynamik von Pb/Si(111)
The unoccupied electronic structure of epitaxial Pb films on Si(1 1 1) is analyzed by angle-resolved two- photon photoemission in the GammaBar → MBar 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_|| ) of the unoccupied quantum well states is weaker than expected for freestanding films, in good agreement with their occupied counterparts. Following E(k_||) 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 6pz 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 GammaBar 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.