Spectral broadening in phonon Raman scattering and its stokes to anti-stokes intensity ratio

We measured the Stokes to anti-Stokes intensity ratio in the phonon Raman spectrum of a rutile (TiO2) crystal. We found that although the ratio still equals the Boltzmann factor at the center frequency ω0 of a mode, it deviates from the Boltzmann factor by up to 5% and approaches a constant value of eħω0/kBT within the spectrally-broadened region. It is suggested that irreversible dissipation is essential to understanding this deviation and the broadening of the phonon mode.