Picosecond infrared double resonance studies on pentafluorobenzene

Utilizing synchronized 50 ps pulses from two independently tunable CO2 lasers, a pump–probe experiment is performed on pentafluorobenzene. The molecule, which has two infrared active modes accessible to the CO2 laser wavelengths, allows an extensive investigation into the statistical nature of the intramolecular vibrational energy relaxation process. We find discrete state effects, indicated by oscillatory behavior in the time-resolved absorption spectrum, disappear when the molecule is heated into the quasicontinuum. Using the anharmonic redshifting and broadening of the normal mode absorption features in the infrared spectrum as a measure of local temperature, our results indicate rapid equilibration (<50 ps) of absorbed energy among modes, with the final distribution of energy consistent with thermal heating.