Intramolecular vibrational energy redistribution and intermolecular energy transfer of benzene in supercritical CO2: measurements from the gas phase up to liquid densities

Femtosecond pump probe spectroscopy was employed to measure intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of benzene in the gas phase and in supercritical (sc) CO2. We observe two IVR time scales the faster of which proceeds within τIVR(1)<0.5 ps. The slower IVR component has a time constant of τIVR(2)=(48±5) ps in the gas phase and in scCO2 is accelerated by interactions with the solvent. At the highest CO2 density it is reduced to τIVR(2)=(6±1) ps. The corresponding IVR rate constants show a similar density dependence as the VET rate constants. Model calculations suggest that both quantities correlate with the local CO2 density in the immediate surrounding of the benzene molecule.