Softening of phonon modes in C60 crystals induced by laser irradiation: Thermal effects

Abstract

Reversible softening of the intramolecular A g(2) pentagonal pinch (PP) mode of a C60 single crystal in the face centered cubic phase has been studied as a function of laser power density by means of Raman scattering. The average temperature rise in the laser excitation spot has been determined using 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 agreement with experimental results obtained for uniformly heated samples. These findings are in excellent agreement with results obtained by numerical calculations of the local temperature distribution and average temperature in the laser spot based on calculated integrated intensities of the Stokes and anti-Stokes bands of the PP-mode. These calculations were based on experimental data for the temperature dependence of phonon frequency and width, absorbance, and thermal conductivity in solid C60.