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 spectr oscopy. 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 distrib utions 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 exci tation of pyrazine by the excimer laser pulse. In addition, the CO2 tr anslational recoil velocity was measured for a number of rotational le vels in each vibrational state. The results of these experiments revea l that very little rotational and translational excitation accompanies the energy transfer from hot pyrazine to excited vibrational levels o f CO2. In contrast, rotational excitation of the CO2 ground state due to collisions with highly excited pyrazine is significant and is accom panied by a substantial enhancement in the CO2 translational energy. T hese results are consistent with a picture in which vibration-vibratio n (V --> V) energy transfer processes, leading to vibrational excitati on of the bath, are dominated by long range attractive forces, and vib ration-translation/rotation (V --> T/R) energy transfer, which leaves the bath vibrations unexcited, is dominated by short range repulsive f orces.