J. H. Weaver

We have determined the optical constants in the energy range 0.1–6 eV for bulk Cu, Ag, and Au using Kramers–Kronig analysis of previously unpublished reflectance data. The results are compared to those commonly used from the literature.

Synchrotron-radiation and x-ray photoemission studies of the valence states of condensed phase-pure ${\mathrm{C}}_{60}$ showed seventeen distinct molecular fetaures extending \ensuremath{\sim}23 eV below the highest occupied molecular states with intensity variations due to matrix-element effects involving both cluster and free-electron-like final states. Pseudopotential calculations established the orign of these features, and comparison with experiment was excellent. The sharp C 1s main line indicated a single species, and the nine satellite structures were due to shakeup and plasmon features. The 1.9-eV feature reflected transitions to the lowest unoccupied molecular level of the excited state.

Electronic-structure studies of ${\mathrm{C}}_{60}$ condensed on metal surfaces show that the energy levels derived from the fullerene align with the substrate Fermi level, not the vacuum level. For thick layers grown on metals at 300 K, the binding energy of the C 1s main line was 284.7 eV and the center of the band derived from the highest occupied molecular orbital was 2.25 eV below the Fermi level. For monolayer amounts of ${\mathrm{C}}_{60}$ adsorbed on Au and Cr, however, the C 1s line was broadened asymmetrically and shifted to lower binding energy, the shakeup features were less distinct, and a band derived from the lowest unoccupied molecular orbital (LUMO) was shifted toward the Fermi level. These monolayer effects demonstrate partial occupancy of a LUMO-derived state, dipole formation, and changes in screening that are associated with LUMO occupancy. Results for ${\mathrm{C}}_{60}$ monolayers on n-type GaAs(110) show transfer of \ensuremath{\le}0.02 electron per fullerene, as gauged by substrate band bending. For ${\mathrm{C}}_{60}$ on p-type GaAs, however, the bands remained flat because electron redistribution was not possible, and the ${\mathrm{C}}_{60}$-derived energy levels were aligned to the substrate vacuum level.

The recent report of electrical conductivity in the alkali metal fullerides and the discovery of superconductivity at 18 K for KxC(60) has raised fundamental questions about the electronic states on either side of the Fermi level, their occupancy with K intercalation, and the mechanism of superconductivity. Direct photoemission evidence is presented of filling of bands derived from the lowest unoccupied molecular orbital as a function of K incorporation for the metallic and insulating phases. This filling is not rigid band-like, and it reflects disorder in the K sites. Theoretical analysis indicates that KxC(60) is a strong coupling superconductor, and we suggest that the enhanced electron-phonon interaction is related to the unique hybridization of the C sp-derived states.

Studies of C(60) overlayer growth on GaAs(110) with scanning tunneling microscopy show large first monolayer islands that are locally well ordered, structurally stable, and commensurate with the GaAs surface owing to molecule-substrate interactions. Within the distorted close-packed structure, two distinct adsorption sites were identified, one of them being elevated because of stress in the C(60) monolayer.