SOME ROTATIONS LIKE IT HOT - SELECTIVE ENERGY PARTITIONING IN THE STATE-RESOLVED DYNAMICS OF COLLISIONS BETWEEN CO2 AND HIGHLY VIBRATIONALLY EXCITED PYRAZINE

The collisional quenching of highly vibrationally excited pyrazine by CO2 molecules has been studied with high resolution diode laser spectroscopy. The vibrationally hot pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. The nascent rotational population distributions in the 00(0)0 and 00(0)1 vibrational levels of CO2 produced by collisions with hot pyrazine were probed at short times following excitation of pyrazine by the excimer laser pulse. In addition, the CO2 translational recoil velocity was measured for a number of rotational levels in each vibrational state. The results of these experiments reveal that very little rotational and translational excitation accompanies the energy transfer from hot pyrazine to excited vibrational levels of CO2. In contrast, rotational excitation of the CO2 ground state due to collisions with highly excited pyrazine is significant and is accompanied by a substantial enhancement in the CO2 translational energy. These results are consistent with a picture in which vibration-vibration (V --> V) energy transfer processes, leading to vibrational excitation of the bath, are dominated by long range attractive forces, and vibration-translation/rotation (V --> T/R) energy transfer, which leaves the bath vibrations unexcited, is dominated by short range repulsive forces.