RAMAN-SPECTROSCOPY OF C-60 SOLID FILMS

In spite of the simplicity of the Raman spectrum of C-60, controversy remains concerning the identification of the Raman spectrum associated with the pure 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 C-60 film as a function of the laser power is reported. As previously reported, two distinct Raman spectra are found: a 1469-cm(-1) Raman spectrum under a low incident laser power (P-lambda=1 mW) and a 1459-cm(-1) Raman spectrum under a moderate incident laser power (P-lambda=50 mW). The temperature dependences of these two Raman spectra in a large temperature range 10-430 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 C-60 under the laser irradiation featured by a dominant 1459-cm(-1) Raman spectrum at room temperature, the 1469-cm(-1) Raman spectrum being assigned to the pure oxygen-free C-60. Our point of view is confirmed by the observation, after a peculiar thermal treatment, of a stable-in-air C-60 form featured by a 1459-cm(-1) room-temperature Raman spectrum. Obviously, this result argues against the interpretation of the 1459-cm(-1) spectrum as that of a pure oxygen-free C-60 solid.