Effect of oxygen on the structural phase transition of solid C₆₀studied by Raman scattering spectroscopy

The Raman spectra of C₆₀films with and without oxygen exposure have been measured in the temperature range 18-300 K in order to investigate the effect of oxygen on the structural phase transition at a temperature near 250 K. While the film without oxygen exposure exhibits a drastic change in the Raman spectrum at the phase transition, the change is much less pronounced for the film with oxygen exposure. The effect of oxygen on the Raman spectra in the low-temperature phase was not seen in contrast with the high-temperature phase in which two A_gvibrational modes are strongly enhanced after oxygen exposure. The observations indicate the importance of the oxygen effect as well as the effect of condensation of C₆₀molecules into the solid forms in understanding the vibrational modes in solid C₆₀. The enhanced Raman modes in the low-temperature phase and oxygen-diffused high-temperature phase are discussed in the light of the resonance with the electronic states of the solid forms. The thermal activation energy for the photodissociation and diffusion of oxygen molecules out of the f.c.c. lattice of C₆₀is estimated to be 0-24 eV through the laser irradiation experiments. The results also suggest the formation of a passive layer on the surface of the oxygen-exposed C₆₀film by laser irradiation.