The geometric and electronic structures of C 60 adsorption on Au͑111͒ surfaces have been studied by lowenergy electron diffraction ͑LEED͒, angle-resolved photoemission, and near-edge x-ray-absorption spectroscopy. An irreversible structural transition of the C 60 overlayer on Au͑111͒ was observed by LEED upon successive annealing. These structures are 38ϫ38 ''in phase,'' R14°and (2)ϫ2))R30°, with the latter phase predominating after annealing to 350°C. Valence-band photoemission spectra reveals a state right below the Fermi level for an annealed, ordered monolayer. This peak disperses across the Fermi energy that indicates the C 60 overlayer becomes metallic. Its intensity shows a resonance that primarily follows the behavior of highest occupied molecular orbitals, identified unambiguously as lowest unoccupied molecular orbitals ͑LUMO's͒ filled by charge transfer from the substrate. An asymmetric distribution of LUMO charge is observed. The thermal-desorption energy of the monolayer is estimated from annealing experiments to be 1.9 eV, which is 0.5 eV larger than the desorption energy from multilayers. Comparison with available spectroscopic data indicates that interaction of C 60 with Au͑111͒ is slightly weaker than with Au͑110͒, and much weaker than with Cu͑111͒. The amount of charge transfer estimated from photoemission is 0.8 electrons per C 60 molecule on Au͑111͒, compared to 1.6 electrons on Cu͑111͒. We argue that charge transfer is determined by the bulk sp density of states at the Fermi energy scaled by the size of the C 60 molecule, and also modified by a clean surface electronic structure, and that charge transfer is the dominant interaction in these systems.

We have carried out an extensive study of C 60 adsorption on Cu͑111͒ surfaces using low-energy electron diffraction, photoemission, and x-ray-absorption spectroscopy. It is found that in valence-band photoemission a state forms right below the Fermi energy for an annealed, well-ordered monolayer, similar to the case of K-doped C 60 . This peak disperses across the Fermi energy at off normal emission geometry. The spectra of carbon core-level photoemission show that the line shape is highly asymmetric with a metalliclike tail. The carbon near-edge absorption spectra show that the lowest unoccupied molecular orbital ͑LUMO͒ is attenuated, and a clear Fermi edge jump appears at the absorption onset. This evidence indicates that charge transfers from the substrate to the C 60 molecular orbitals and the overlayer becomes metallic. The amount of charge transfer can be determined to be 1.5-2 electrons per molecule from both the area of the occupied LUMO in photoemission and the peak shift in near-edge absorption spectra. It has been reported that many metal surfaces with originally different work functions covered by a monolayer of C 60 have a similar work function of about 5 eV. We suggest that the measured work functions are due to the metallic C 60 overlayers and are similar regardless of the metal substrates. This is in line with the reported alignment of monolayer energy levels to substrate Fermi energy. Since the work functions are similar, the energy levels with respect to the vacuum level are also similar. Finally we compare near-edge x-ray-absorption with inverse photoemission spectroscopy to address the screening effects.

The insulating Be 2 C thin films have been successfully prepared on a Be surface. A low-energy electron diffraction pattern shows that the films have ͑100͒ orientation along the surface normal. We have used angleresolved photoemission to map out the occupied bulk band dispersion along the ⌫-X direction. The band-gap edges at the X point are 6.7 and 11.5 eV below the valence-band maximum, which is located at the ⌫ point. These values are in good agreement with theoretical calculations. The unoccupied bulk electronic structure is measured using C 1s near-edge x-ray-absorption spectroscopy. The spectrum is similar in shape to the energyloss spectrum and the calculated p-partial density of states, while the peak positions are different. ͓S0163-1829͑98͒01335-6͔

We have investigated the bonding nature and hole-electron Coulomb interaction U in thin C(60) films on Be(0001) surfaces using valence-band and core-level photoemission, inverse photoemission, and near-edge x-ray absorption spectroscopies. The C(60) monolayer had strong covalent bonding with the Be substrate, producing a nearly insulating film, in contrast to a metallic overlayer due to charge transfer observed on many other metallic surfaces. The effect of polarization of surrounding molecules and the image potential decreases the energy gap and U, but the bonding-antibonding contribution increases the gap at the interface. The measured U in thin solid films agrees well with a model calculation using gas-phase values. The deduced hole-electron attraction on the surface is about 0.7 eV larger than the reported hole-hole repulsion determined by Auger spectroscopy. On the basis of the surface-solid difference, the newly estimated value of U for hole-hole correlation places doped C(60) compounds nearer the metallic side of a Mott transition.

Highly oriented crystalline beryllium carbide films were grown on Be͑0001͒ substrate using C 60 as a carbon source. The films were characterized by low energy electron diffraction, photoemission spectroscopy, and near-edge x-ray absorption fine structure. C 60 begins to decompose on Be͑0001͒ at about 250°C, forming beryllium carbide completely after further annealing to 450°C. The beryllium carbide film is observed as sets of ͑100͒ surfaces, arranged in three domains rotated by 120°from each other. Extra C 60 deposited on Be 2 C(100)/Be(0001) at temperature below 200°C and heated to 450°C leads to an increase of the film thickness, indicating the decomposition of C 60 on Be 2 C(100)/Be(0001) at an elevated temperature and formation of new carbide layers on the sample surface. It further implies that the Be 2 C/Be surface has the ability to supply Be atoms to interact with the new carbon atoms on top, and that Be atoms can diffuse through the beryllium carbide layer at the temperature of 450°C, possibly involving a vacancy mechanism.