Raman Spectroscopy of C60 Solid Films: A Tale of Two Spectra

Abstract

Two distinct Raman spectra have been reported for solid C[sub 60]. They differ in the exact position and relative intensities of the spectral lines. Specifically, the frequency of the pentagonal pinch mode occurs at 1469 and 1459 cm[sup [minus]1] in the two spectra. Several explanations have been offered for the existence of a second spectrum, including the influence of interstitial oxygen, a change in phase, the coexistence of singlet C[sub 60] with laser-pumped triplet C[sub 60], and photopolymerization. We show that there are two Raman spectra for solid films of C[sub 60] with varying relative intensities according to the sample temperature even in the absence of oxygen. The reversibility of the two forms of C[sub 60] with temperature argues against photopolymerization. Finally pump-probe experiments designed to detect either triplet absorption or singlet depletion indicate that neither of the two Raman spectra are due to triplet C[sub 60]. We conclude that our previous explanation in terms of two phases of C[sub 60] is still the most consistent with the observations. 28 refs., 10 figs.