Fast collision-induced redistribution of vibrational energy in halogenated methanes Original Research Article

Abnormally fast relaxation of the high-frequency CH stretch mode of CHF2Cl and CHF3 molecules in mixtures with different buffer gases is observed. The probability of this process with respect to hard-sphere collision rate reaches 0.38 for the noble buffer gas (CHF2Cl:Kr mixture) and 1.03 for the polar buffer gas (CHF2Cl:SO2 mixture). This result is referred to the collision-induced intramolecular vibrational relaxation (CIIVR) leading to the redistribution of vibrational energy from the initially excited mode among the nearby optically ‘dark’ states. The mechanisms resulting in CIIVR are discussed. It is shown that the high efficiency of this process results from the joint action of intra- and intermolecular interactions. The number of optically ‘dark’ vibrational states involved in CIIVR is experimentally measured for the CHF2Cl molecule. The obtained value coincides with the number of three-frequency combination states in the vicinity of the initially excited state.