FAST COLLISION-INDUCED REDISTRIBUTION OF VIBRATIONAL-ENERGY IN HALOGENATED METHANES

Abnormally fast relaxation of the high-frequency C-H stretch mode of C HF2Cl and CHF3 molecules in mixtures with different buffer gases is ob served. The probability of this process with respect to hard-sphere co llision 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 i s referred to the collision-induced intramolecular vibrational relaxat ion (CIIVR) leading to the redistribution of vibrational energy from t he initially excited mode among the nearby optically 'dark' states. Th e mechanisms resulting in CIIVR are discussed. It is shown that the hi gh efficiency of this process results from the joint action of intra- and intermolecular interactions. The number of optically 'dark' vibrat ional states involved in CIIVR is experimentally measured for the CHF2 Cl molecule. The obtained value coincides with the number of three-fre quency combination states in the vicinity of the initially excited sta te. (C) 1997 Published by Elsevier Science B.V.