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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.

This work presents state-resolved measurements of weak and strong collisions between HOD and highly vibrationally excited pyrazine (Evib = 37,900 cm(-1)). Transient IR absorption line profiles of HOD(000) rotational states that are populated in single collisions with pyrazine (Evib) are fit using double-Gaussian functions to extract Doppler-broadened line widths and energy transfer rates for appearance and depletion populations. We recently reported the use of this new approach to determine collision rates for energy transfer (Havey, D. K.; Liu, Q.; Li, Z. M.; Elioff, M.; Fang, M.; Neudel, J.; Mullin, A. S. J. Phys. Chem. A 2007, 111, 2458-2460). Here we present a full description of transient measurements of weak collisions, along with rate constants and the full energy transfer distribution function for the vibrational-to-rotation/translation (V-RT) pathway. The low- and high-J populations of scattered HOD(000) are characterized by a single rotational distribution with Trot = 430 +/- 50 K. The average translational energy of the scattered HOD(000) increases modestly with rotational energy suggesting an impulsive energy transfer mechanism. The energy gain distribution P(Delta E) for HOD(000) shows that approximately 99% of collisions have DeltaE values less than 3000 cm(-1). These data yield a lower limit to the collision rate that is approximately 70% larger than the calculated Lennard-Jones collision rate. These findings show that water is an efficient quencher due to a large collisional energy transfer cross section and not as a result of a large supercollision tail.

The role of alkylation on the energy partitioning in strong collisions with CO2 was investigated for highly vibrationally excited 2-ethylpyridine (2EP) and 2-propylpyridine (2PP) prepared with E(vib) approximately 38,570 and 38,870 cm(-1), respectively, using lambda = 266 nm light. Nascent energy gain in CO2 (00(0)0) rotation and translation was measured with high-resolution transient absorption spectroscopy at lambda approximately 4.3 microm and the results are compared to earlier relaxation studies of pyridine (E(vib) = 37,950 cm(-1)) and 2-methylpyridine (2MP, Evib = 38,330 cm(-1)). Overall, the alkylated donors impart less rotational and translational energy to CO2 than does pyridine. 2PP consistently imparts more translational energy in collisions than does 2EP and has larger energy transfer rates. Of the alkylated donors, 2MP and 2PP have larger probabilities for strong collisional energy transfer than does 2EP. Two competing processes are discussed: donors with longer alkyl chains have lower average energy per mode and fewer strong collisions but longer alkyl chains increase donor flexibility, leading to higher state densities that enhance energy loss via strong collisions. A comparison of state density effects based on Fermi's Golden Rule shows that 2PP has more strong collisions than predicted while 2EP has fewer. The role of torsional motion in the hot donors is considered. Comparison of effective impact parameters shows that the alkylated donors undergo strong collisions with CO2 via a less repulsive part of the intermolecular potential than does pyridine.

Classical trajectory calculations were performed to simulate state-resolved energy transfer experiments of highly vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) and CO(2), which were conducted using a high-resolution transient infrared absorption spectrometer. The goal here is to use classical trajectories to simulate the supercollision energy transfer pathway wherein large amounts of energy are transferred in single collisions in order to compare with experimental results. In the trajectory calculations, Newton's laws of motion are used for the molecular motion, isolated molecules are treated as collections of harmonic oscillators, and intermolecular potentials are formed by pairwise Lennard-Jones potentials. The calculations qualitatively reproduce the observed energy partitioning in the scattered CO(2) molecules and show that the relative partitioning between bath rotation and translation is dependent on the moment of inertia of the bath molecule. The simulations show that the low-frequency modes of the vibrationally excited pyrazine contribute most to the strong collisions. The majority of collisions lead to small DeltaE values and primarily involve single encounters between the energy donor and acceptor. The large DeltaE exchanges result from both single impulsive encounters and chattering collisions that involve multiple encounters.

Supercollision relaxation of highly vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) with D35Cl is investigated using high-resolution transient IR diode laser absorption spectroscopy at 4.4 microm. Highly excited pyrazine is prepared by pulsed UV excitation at 266 nm, followed by rapid radiationless decay to the ground electronic state. The rotational energy distribution of the scattered DCl (v = 0,J) molecules with J = 15-21 is characterized by T(rot) = 755+/-90 K. The relative translational energy increases as a function of rotational quantum number for DCl with T(rel) = 710+/-190 K for J = 15 and T(rel) = 1270+/-240 K for J = 21. The average change in recoil velocity correlates with the change in rotational angular momentum quantum number and highlights the role of angular momentum in energy gain partitioning. The integrated energy-transfer rate for appearance of DCl (v = 0,J = 15-21) is k(2)(int) = 7.1x10(-11) cm3 molecule(-1) s(-1), approximately one-eighth the Lennard-Jones collision rate. The results are compared to earlier energy gain measurements of CO2 and H2O.