RAMAN-SPECTROSCOPY OF C-60 SOLID FILMS - A TALE OF 2 SPECTRA

Two distinct Raman spectra have been reported for solid C-60 They diff er in the exact position and relative intensities of the spectral line s. Specifically, the frequency of the pentagonal pinch mode occurs at 1469 and 1459 cm(-1) in the two spectra. Several explanations have bee n offered for the existence of a second spectrum, including the influe nce of interstitial oxygen, a change in phase, the coexistence of sing let C-60 with laser-pumped triplet C-60, and photopolymerization. We s how that there are two Raman spectra for solid films of C-60 with vary ing relative intensities according to the sample temperature even in t he absence of oxygen. The reversibility of the two forms of C-60 With temperature argues against photopolymerization. Finally pump-probe exp eriments designed to detect either triplet absorption or singlet deple tion indicate that neither of the two Raman spectra are due to triplet C-60. We conclude that our previous explanation of C-60 is still the most consistent with the observations.