XPS study of a photopolymerized C-60 film

The changes in the valence band and the C 1s core peak and its shake-up satellite of a C-60 film upon photopolymerization are studied by x-ray photoelectron spectroscopy. It was found that the intensity of the C 1s shake-up satellite of the phototransformed C-60 film decreased with increasing irradiation time of a 500-W mercury lamp (0-200 h). On the other hand, the valence-band structure of the C-60 film was almost unchanged even after 200 h of irradiation, compared with that of the pristine C-60 film. This suggests that the change in the transition probability of each C 1s shake-up satellite peak (pi-pi* transitions) is very small after irradiation and thus plays only a minor role in the reduction of the satellite intensity. The average number of [2 + 2] cycloadditional four-membered ring structures on a C-60 molecule was estimated from the decrease in the satellite intensity.