Collisional deactivation of a selected energy level in S₀ p-difluorobenzene embedded in a dense vibrational field: Absolute rate constants for a variety of collision partners

Absolute rate constants are reported for collisional relaxation from the 5₂30₂ level (ε_vib=2036 cm^-1) of S₀(¹A_g)p-difluorobenzene into the relatively dense surrounding field of S₀ vibrational levels (6-8 per cm ^-1). The relaxation is induced by a bath of foreign gas at 300 K under bulb conditions. A total of 23 collision partners have been studied. Stimulated emission pumping (SEP) is used to populate the 5₂30 ₂ level selectively. Single vibronic level fluorescence (SVLF) from the S₁ level 30², generated by exciting the 5 ₂⁰30₂² transition at a variable delay time (85-260 ns) after 5₂30₂ preparation, is used to monitor the population dynamics of 5₂30₂. The equivalent cross sections deduced from the rate constants for vibrational relaxation with foreign gas M range from one quarter of the hard sphere value for M=He to 2.7 times hard sphere for M=(C₂H₅)₂O. These are an order of magnitude larger than the cross sections normally found for vibrational relaxation in ground electronic states of smaller molecules, however, they correlate well with cross sections observed in excited states of related aromatics. The absolute magnitude of the vibrational relaxation rate constants observed for S₀ p DFB are well reproduced for all but the lightest collision partners (He, H₂, and D₂) by a model based on the Lennard-Jones collision rate coefficient. We conclude that the Lennard-Jones collision frequency, or collision frequencies calculated using similar realistic intermolecular potentials, are a good representative model for calculating vibrational relaxation rates in regions of high vibrational state density. Deviation from the commonly used hard sphere rate coefficient is expected to be most marked at low temperatures (<150 K), but even at 300 K, the hard sphere rate is found to underestimate the vibrational relaxation rate by a factor of 2 for vibrationally complex collision partners.