Raman spectroscopy of C 60 solid films

In spite of the simplicity of the Raman spectrum of ${\mathrm{C}}_{60}$, controversy remains concerning the identification of the Raman spectrum associated with the pure ${\mathrm{C}}_{60}$ solid. In particular, the exact frequency of the pentagonal pinch mode is still an open question. In this paper, the change of the room-temperature Raman spectrum of an oxygen-free ${\mathrm{C}}_{60}$ film as a function of the laser power is reported. As previously reported, two distinct Raman spectra are found: a 1469-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ Raman spectrum under a low incident laser power (${\mathit{P}}_{\ensuremath{\lambda}}$=1 mW) and a 1459-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ Raman spectrum under a moderate incident laser power (${\mathit{P}}_{\ensuremath{\lambda}}$=50 mW). The temperature dependences of these two Raman spectra in a large temperature range 10\char21{}430 K, and under low, moderate, and high incident laser power are analyzed. The most consistent explanation of these dependences is a phototransformation of oxygen-free ${\mathrm{C}}_{60}$ under the laser irradiation featured by a dominant 1459-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ Raman spectrum at room temperature, the 1469-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ Raman spectrum being assigned to the pure oxygen-free ${\mathrm{C}}_{60}$. Our point of view is confirmed by the observation, after a peculiar thermal treatment, of a stable-in-air ${\mathrm{C}}_{60}$ form featured by a 1459-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ room-temperature Raman spectrum. Obviously, this result argues against the interpretation of the 1459-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ spectrum as that of a pure oxygen-free ${\mathrm{C}}_{60}$ solid.