MOLECULAR SUPERCOLLISIONS - EVIDENCE FOR LARGE ENERGY-TRANSFER IN THE COLLISIONAL RELAXATION OF HIGHLY VIBRATIONALLY EXCITED PYRAZINE BY CO2

The temperature dependence of the collisional quenching of highly vibrationally excited pyrazine by CO2 molecules has been investigated for the temperature range 243-364 K using high resolution time resolved diode laser spectroscopy. Particular emphasis is placed on vibration to rotation-translation (V→R/T) energy transfer which leaves the CO 2 vibrations unexcited and occurs predominantly through short-range repulsive forces. Vibrationally hot pyrazine is prepared by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. For the range of experimental cell temperatures used here, the nascent rotational population distributions of the 00°0 ground state of CO 2 resulting from collisions with hot pyrazine were probed at short times following excitation of pyrazine by the excimer laser pulse. The CO 2 translational recoil velocity was also measured for individual rotational levels of the 00°0 state. In addition, temperature dependent rate constants and probabilities were determined for energy transfer from the vibrationally hot pyrazine into individual rotational levels of the 00°0 state of CO2. The rotational distributions, velocity recoils, and quenching rates exhibit a very weak temperature dependence for production of CO2 high J states, indicating that the CO2 molecules involved in these energy transfer events originate from rotational levels only slightly greater than the thermal mean J value. Based on these results, values for ΔE, the energy transfer from hot pyrazine to CO2 resulting in final CO2 00°0 states J=58 through J= 82, are estimated to range from 2550 to 7090 cm-1 in a single collision. © 1995 American Institute of Physics.