A Chemical Timing Method for Absolute Vibrational Relaxation Rate Constants in the Vibrational Quasi-Continuum Region of S1 p-Difluorobenzene

Abstract

A new method has been developed to measure absolute rate constants for gas-phase collisional vibrational relaxation in high S 1 p-difluorobenzene (pDFB) vibrational energy regions where the levels overlap to form a quasi-continuum. The heavily mixed vibrational character of these levels produces S 1 → S 0 fluorescence without the vibrational structure that is needed to measure rate constants. Following well-studied chemical timing procedures, the addition of large pressures (in the kTorr range) of O 2 to a low pressure pDFB sample yields a structured fluorescence spectrum that proves to be suitable for measurement of absolute rate constants for vibrational relaxation induced by argon added to the pDFB + O 2 mixture. The first application concerns S 1 pDFB with ∈ v i b = 3700 cm - 1 and a high vibrational state density (ρ v i b > 4000 per cm - 1 ) that creates a quasi-continuum. The absolute rate constant for vibrational energy transfer in Ar collisions from the initially pumped region into the surrounding vibrational field is k Α r v = (9.4 ′ 1.5) x 10 6 Torr - 1 s - 1 = 2.9 × 10 - 1 0 cm 3 molecule - 1 s - 1 . This rate constant is within the experimental error of rate constants determined previously for three lower levels with ∈ v i b ≥ 2887 cm - 1 . The value is roughly 50% of the Lennard-Jones rate constant usually assumed for the vibrational activation/deactivation step of thermal unimolecular reactions. A rate constant analogous to k A r v but for O 2 collisions is determined to have approximately the same value as k A r v. An approximate intramolecular vibrational redistribution (IVR) rate constant for S 1 pDFB with ∈ v i b = 3700 cm - 1 corresponds to an IVR lifetime in the range of 45-75 ps.