A chemical timing method for absolute vibrational relaxation rate constants in the vibrational quasi-continuum region of S1 p-difluorobenzene

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 is an element of(vib) = 3700 cm(-1) and a high vibrational state density (rho(vib) > 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(v)(Ar) = (9.4 +/- 1.5) x 10(6) Torr(-1) s(-1) = 2.9 x 10(-10) cm(3) molecule(-1) s(-1). This rate constant is within the experimental error of rate constants determined previously for three lower levels with is an element of(vib) greater than or equal to 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(v)(Ar) but for O-2 collisions is determined to have approximately the same value as k(v)(Ar). An approximate intramolecular vibrational redistribution (IVR) rate constant for S-1 pDFB with is an element of(vib) = 3700 cm(-1) corresponds to an IVR lifetime in the range of 45-75 ps.