Intramolecular and intermolecular vibrational energy relaxation of CH2I2 dissolved in supercritical fluid

A pump–probe experiment was performed to examine vibrational population relaxation of diiodomethane (CH2I2) molecule dissolved in supercritical CO2. Using an apparatus with femtosecond time resolution, we observed the contributions of intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) separately. IVR and VET rates were measured with varying solvent densities at a constant temperature. It is shown that the IVR rate is not density dependent while the VET rate increases with increasing density from 0.4 to 0.8 g cm−3. This observation suggests that the rate of the VET process is determined by solute–solvent collisions whereas the IVR rate is not much affected by solute–solvent interaction.