| State-resolved studies of the collisional deactivation of highly vibrationally excited molecules by small collision partners in a low-pressure environment at 300 K have been performed. Vibrationally excited azabenzenes were prepared by electronic excitation using 250--324 nm light followed by rapid radiationless decay to the electronic ground state. High-resolution transient infrared absorption was used to investigate energy gain in CO2 and H2O in several vibrational states. Nascent rotational, vibrational, and translational energy distributions were measured for scattered bath molecules and absolute cross-sections were determined. For collisional quenching of pyrazine by CO2 experiments were designed to investigate donor energy dependence on the excitation of the CO2 (0000), (02 20), (1000R1), (1000 R2), and (0001) vibrational states. The near-resonant energy transfer channel, in which vibrational energy gain in CO2 occurs with little or no accompanying rotational or translational excitation, exhibits essentially no dependence on donor internal energy. The "supercollision" channel, wherein vibrational energy in the donor molecule results in translational and rotational excitation of the CO2 molecule, shows interesting threshold behavior near 36000 cm-1. The collisional deactivation of vibrationally excited pyrazine, pyridine, and methyl pyridines by water was also investigated. For all donors investigated very little recoil energy, but large amounts of rotational energy, in the scattered water molecules was observed. Quenching of vibrationally excited pyridine showed only a slight decrease, relative to pyrazine, in rotational excitation of water. The magnitudes and probabilities for energy transfer to H2O(000) states with E rot > 1000 cm-1 from vibrationally excited pyrazine accounted for average energy losses from pyrazine of ∼30cm-1 per collision, dramatically less than the energy losses of ∼150 cm-1 per collision observed for CO2(0000) with Erot = 1250--2800 cm-1. Studies of methyl pyridine deactivation indicate that the impulsive nature of the collisional energy transfer is diminished as average energy per mode decreases. Collisional quenching of pyrazine by isotopically substituted water was investigated to examine the effect of the relatively small moment of rotational inertia of water in the mechanics of vibrational energy transfer. Velocity distributions for depletion of HDO(000) states with low rotational energies show that translational energy gain is not a significant factor in deactivation by water. |
