Unoccupied electronic structure and relaxation dynamics of Pb/Si(1 1 1)

Highlights: • The electronic structure of unoccupied quantum well states in Pb/Si(1 1 1) is analyzed. • The Si substrate modifies the binding energy and dispersion of the Pb bands. • The direct Pb 6p_z orbital overlap with Si orbitals is marginal. • We conclude a 6p_x,y mediated interaction of the 6p_z states with the Si bulk bands. - Abstract: The unoccupied electronic structure of epitaxial Pb films on Si(1 1 1) is analyzed by angle-resolved two-photon photoemission in the Γ^¯→M^¯ 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 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 Γ^¯ 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