RAMAN STUDIES OF PHOTOCHEMICAL-REACTIONS IN FULLERENE FILMS

Raman spectroscopy is employed as a structural probe of both oxygen-fr ee and oxygenated thin films of C60. Comparison of initial spectra rev eals no influence of oxygen inclusion on the Raman frequencies. The am plitudes of the peaks are, however, observed to be sensitive to light exposure. Each of the initially observed peaks degrades on a time scal e of tens of seconds while new peaks emerge approximately ten wavenumb ers to the low-energy side. In oxygenated films the degradation proces s is apparently slower. The process is analyzed using a simple rate eq uation model of a level scheme in which population of the triplet stat e leads to a sample degradation with a characteristic time tau2, compe ting with the ground state recovery, characterized by the triplet life time, tau1. The decay of all the Raman peaks in oxygen-free films fits well to such a model with values of 80 +/- 5 s and 55 +/- 5 ms for ta u2 and tau1, respectively. The rise of the new spectral features, corr esponding to the degraded product, also fits the model. In oxygenated films, where a similar value of tau2 is found, the best fits yield a t riplet lifetime of 30 +/- 5 ms. It is proposed that the role of oxygen is to inhibit the photochemical degradation via triplet quenching.