Softening of phonon modes in C-60 crystals induced by laser irradiation: Thermal effects

Reversible softening of the intramolecular A(g) (2) pentagonal pinch (PP) m ode of a C-60 single crystal in the face centered cubic phase has been stud ied as a function of laser power density by means of Raman scattering. The average temperature rise in the laser excitation spot has been determined u sing the Stokes to anti-Stokes integrated peak intensity ratio for the H-g (1) phonon mode. Softening of the PP-mode was found to be due to heating of the sample resulting from laser irradiation, in good quantitative agreemen t with experimental results obtained for uniformly heated samples. These fi ndings are in excellent agreement with results obtained by numerical calcul ations of the local temperature distribution and average temperature in the laser spot based on calculated integrated intensities of the Stokes and an tiStokes bands of the PP- mode. These calculations were based on experiment al data for the temperature dependence of phonon frequency and width, absor bance, and thermal conductivity in solid C-60. (C) 1998 American Institute of Physics. [S1063-7761(98)01611-4].