Energy-Dependent Quantum-State-Resolved Relaxation of Highly Vibrationally Excited Pyridine (Evib = 36 990−40 200 cm-1) through Collisions with CO2

The energy dependence of "strong" collisional deactivation that transfers energy from highly vibrationally excited pyridine to a CO 2 bath has been investigated for pyridine with internal energy between E v i b = 36 990 and 40 200 cm - 1 . Highly excited pyridine was prepared by absorption of tunable pulsed UV laser light at four wavelengths, λ = 251, 259, 266, and 273 nm, followed by prompt radiationless decay from S 1 (or S 2 ) to the vibrationally excited levels of the ground electronic state. High-resolution transient IR absorption spectroscopy was used to measure the nascent rotational and translational energy distributions of scattered CO 2 molecules in high J states of the ground vibrationless (00 0 0) level that result from collisions with hot pyridine. Our results reveal that substantial amounts of rotational and translational energy are imparted to CO 2 (00 0 0) molecules for all four pyridine energies and that the magnitude of the CO 2 rotational and translational energy gains changes very little for a 3000 cm - 1 change in the vibrational energy of the pyridine donor. State-resolved energy-transfer rate constants were measured at each UV wavelength and were also found to be fairly insensitive to changes in the donor internal energy content for the excitation energies studied here. These results indicate that the relaxation mechanism and the energy-transfer probability distribution function for the V → RT pathway for pyridine(E)/CO 2 collisions are invariant to this change in internal energy. A consideration of energy-transfer rates using Fermi's Golden Rule is consistent with the observed behavior. Comparison with previous energy-dependent collisional quenching studies [Elioff, M. S.: Wall, M.; Lemoff, A.; Mullin, A. S. J. Chem. Phys. 1999, 110, 5578-5588] on vibrationally hot pyrazine with CO 2 suggests that this may be a general phenomenon in the collisional relaxation of highly excited molecules.