Thermal effect in unoccupied molecular orbitals of C-60 molecules adsorbedon a Si(001)-(2 x 1) surface studied by NEXAFS

We report here the temperature-dependent unoccupied molecular orbitals (MO' s) of C-60 molecules adsorbed on a Si(001)-(2 x 1) surface measured using n ear edge x-ray absorption fine structure (NEXAFS). At 300 K, the NEXAFS spe ctrum reveals that the interaction between a 1.0 monolayer (ML) C-60 film a nd 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-maximu m of unoccupied MO's, which indicates the change of the interaction. Moreov er, the lowest unoccupied molecular orbital (LUMO) shifts to the higher pho ton energy side and the intensity of the LUMO+1 relative to that of the LUM O+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.