Photoemission intensity oscillations in the HOMO-2 band of C60 film

The photoionization cross-sections of the three up-most bands (HOMO, HOMO-1 and HOMO-2) of C60 film have been studied with synchrotron radiation photoelectron spectroscopy. The incident photon energy is from 13.1eV to 179.1eV. Besides the well-known intensity oscillations of the HOMO and HOMO-1 bands, the HOMO-2 band also exhibits oscillations. The oscillations of the three bands have identical periods. However, the HOMO-2 oscillations are apparent only for photon energy hυ<130eV and the oscillating amplitude around 85eV is particularly attenuated. We ascribe the HOMO-2 oscillations to the π electron components. The σ electrons of the HOMO-2 band do not exhibit distinct intensity oscillation because of the σ components generated by two partial waves differing by one in photoelectron angular momentum and hence compensating each other. The compensation is not entire due to the nonspherical part of the potential, which is responsible for the attenuations and irregular background observed in the HOMO-2 oscillations. The results can be used to test the validity of various theoretical models and help to reveal the mechanism of the cross-section oscillations of C60.