Electronic and geometric structure ofC60on Al(111) and Al(110)

Two new ordered monolayer phases of ${\mathrm{C}}_{60}$ on Al surfaces have been studied using electron spectroscopies and low-energy electron diffraction (LEED). On Al(111), in addition to the previously reported (6×6) phase formed by evaporating with ${\mathrm{T}}_{\mathrm{sample}}$=620 K, a metastable $(2\mathrm{}\sqrt{3}\times 2\sqrt{3})R30\mathrm{}°$ phase can be produced with ${\mathrm{T}}_{\mathrm{sample}}$=300 K. This phase exhibits the first LEED pattern reported for an unannealed ${\mathrm{C}}_{60}$ overlayer. On Al(110), when evaporations are also made with ${\mathrm{T}}_{\mathrm{sample}}$=620 K, LEED shows the presence of a monolayer with a pseudo-$c(4\mathrm{}\times 4)$ structure. Al $2p$ photoemission for ${\mathrm{C}}_{60}$/Al(110) and for $(2\mathrm{}\sqrt{3}\times 2\sqrt{3})R30\mathrm{}°$ ${\mathrm{C}}_{60}$/Al(111) reveals no evidence of strong substrate reconstruction. The perturbation of the geometric and electronic structure of the ${\mathrm{C}}_{60}$ molecule due to the bonding interaction with the Al surface increases in the order ${\mathrm{C}}_{60}$/Al(110), $(2\mathrm{}\sqrt{3}\times 2\sqrt{3})R30\mathrm{}°$ ${\mathrm{C}}_{60}$/Al(111), (6×6) ${\mathrm{C}}_{60}$/Al(111), as demonstrated using element-specific probing of the valence band with x-ray absorption and C $1s$ shakeup. The bond has covalent character in all three cases. Symmetry-induced splitting in the ${5h}_u$-derived level is observed using valence photoemission, and is particularly clear for ${\mathrm{C}}_{60}$/Al(110). The stability of the equilibrium structures can be qualitatively understood from considerations of the energetics of the overlayer compression and the chemical bond between adsorbate and substrate. Work-function measurements for these and other ${\mathrm{C}}_{60}$ overlayer systems cannot, in general, be understood within a simple description involving the addition of a dipole to the surface potential.