Bonding state of theC60molecule adsorbed on aSi(111)−(7×7)surface

We report here the measurements of the valence spectra, the C $1s$ and the Si $2p$ core-level spectra of ${\mathrm{C}}_{60}$ molecules adsorbed on a $\mathrm{Si}\mathrm{}\left(111\right)-(7\mathrm{}\times 7)$ surface, using photoelectron spectroscopy. In the valence spectra, the highest occupied molecular orbital (HOMO) of a ${\mathrm{C}}_{60}$ splits into two peaks at a coverage lower than 0.25 ML. The binding energies of the split peaks are 1.8 and 2.4 eV. Taking into account the polarization-dependence of the valence spectra and the binding energy of the C $1s$ core-level spectra, it is found that the 2.4-eV peak observed in the valence spectra is the covalent bonding state between a ${\mathrm{C}}_{60}$ molecule and the Si substrate, and that the 1.8-eV peak is the shifted HOMO. The Si $2p$ core-level spectra suggest that the bonding site is localized at the interface. We also present the energy-level scheme of the bonding state in terms of the symmetry of the HOMO.