Raman studies of photochemical reactions in fullerene films

Raman spectroscopy is employed as a structural probe of both oxygen-free and oxygenated thin films of C₆₀. Comparison of initial spectra reveals no influence of oxygen inclusion on the Raman frequencies. The amplitudes of the peaks are, however, observed to be sensitive to light exposure. Each of the initially observed peaks degrades on a time scale of tens of seconds while new peaks emerge approximately ten wavenumbers to the low-energy side. In oxygenated films the degradation process is apparently slower. The process is analyzed using a simple rate equation model of a level scheme in which population of the triplet state leads to a sample degradation with a characteristic time τ₂, competing with the ground state recovery, characterized by the triplet lifetime, τ₁. 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 τ₂ and τ₁, respectively. The rise of the new spectral features, corresponding to the degraded product, also fits the model. In oxygenated films, where a similar value of τ₂ is found, the best fits yield a triplet lifetime of 30 ± 5 ms. It is proposed that the role of oxygen is to inhibit the photochemical degradation via triplet quenching.