Thermal effect in unoccupied molecular orbitals of C₆₀ molecules adsorbed on a Si(001)-(2 × 1) surface studied by NEXAFS

We report here the temperature-dependent unoccupied molecular orbitals (MO's) of C₆₀ molecules adsorbed on a Si(001)-(2 × 1) surface measured using near edge x-ray absorption fine structure (NEXAFS). At 300 K, the NEXAFS spectrum reveals that the interaction between a 1.0 monolayer (ML) C₆₀ film and a Si(001) surface is mainly the van der Waals force. After annealing the samples at 500 K, we observe an increment in the full-width at half-maximum of unoccupied MO's, which indicates the change of the interaction. Moreover, the lowest unoccupied molecular orbital (LUMO) shifts to the higher photon energy side and the intensity of the LUMO+1 relative to that of the LUMO+3 decreases in the NEXAFS spectrum. These results suggest that the strong interaction induced at 500 K has a covalent character, to which the LUMO+1 contributes.