On the nature of the laser irradiation induced reversible softening of phonon modes in C60 single crystals

The average temperature rise in the laser excitation spot of the C60 single crystal has been determined using the Stokes to anti-Stokes integrated peak intensity ratio for the Hg(1) phonon mode. The reversible softening of the Ag(2) pentagon pinch mode was found to be due to the heating of the sample caused by the laser irradiation, in agreement with experimental results obtained for uniformly heated samples. These findings are in quantitative agreement with results obtained by numerical calculations of the local temperature rise, which indicates the highly non-uniform temperature distribution in the laser excitation spot associated with the small thermal conductivity of solid C60.