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Collision rates and energy transfer distributions are reported for HOD with highly vibrationally excited 2-methylpyridine (2-picoline, E = 38 310 cm(-1)) and 2,6-dimethylpyridine (2,6-lutidine, E = 38 700 cm(-1)). High resolution transient IR absorption is used measured to complete product state distributions of scattered HOD(000) molecules with E(rot) = 109 to 1331 cm(-1). Doppler-broadened line profiles characterize the depletion and appearance for HOD molecules due to collisions with hot donors and show that the product translational and rotational energy distributions are similar for both donors with DeltaE(rel) = 370 cm(-1) and DeltaE(rot) approximately 75 cm(-1). The energy transfer rate for picoline (E)/HOD is 2.5 times larger than the Lennard-Jones collision rate. The energy transfer rate for lutidine(E)/HOD is 3.2 times larger than the Lennard-Jones collision rate. Previous work ( Havey, Liu, Li, Elioff, and Mullin, J. Phys. Chem. A 2007, 111, 13321-9 ) reported similar energy transfer values for pyrazine/HOD collisions and an energy transfer rate that is 1.7 times the Lennard-Jones collision rate. The observed collision rates are discussed in terms of hydrogen bonding interactions between HOD and the aromatic donor molecules. Energy gain profiles for HOD are compared with those for H(2)O.

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.

We describe a new approach for measuring absolute rates for molecular collisions including contributions of both strong and weak collisions. Elastic and inelastic collisions are monitored using high-resolution transient IR spectroscopy by measuring increases in the velocity distributions of individual rotational states of scattered molecules. Weak collisional energy transfer is detected by measuring velocity increases for the low-energy rotational states. This technique is illustrated for the collisional relaxation of highly vibrationally excited pyrazine (108 kcal/mol) with HOD. The observed collision rate is nearly twice the Lennard-Jones collision rate.