Thermal-dependent unoccupied electronic structure of a C-60 monolayer filmadsorbed on a Si(111)-(7 x 7) surface

We have measured the unoccupied electronic structures of a C-60 monolayer f ilm adsorbed on a Si(111)-(7 x 7) surface at 300 and 500 K, using inverse p hotoemission spectroscopy (IPES) and near edge X-ray absorption fine struct ure (NEXAFS). At 300 K, both IPES and NEXAFS spectra reveal that most of C, ,) molecules interact with the Si surface by van der Waals force at a cover age of 1.0 monolayer. After annealing the sample at 500 K, the full-width a t half maximum of the unoccupied molecular orbitals becomes broader indicat ing the chemisorption at this temperature. Moreover, the NEXAFS spectrum sh ows the shift of the lowest unoccupied molecular orbital (LUMO) to the high er photon energy side and the decrease in intensity of the LUMO + 1. These results suggest that the strong interaction induced at 500 K has a covalent character, to which the LUMO + I contributes. (C) 2001 Elsevier Science B. V. All rights reserved.