SUPERCOLLISIONS, PHOTOFRAGMENTATION AND ENERGY-TRANSFER IN MIXTURES OF PYRAZINE AND CARBON-DIOXIDE

The quantum yield for the formation of HCN from the photodissociation of pyrazine excited at 248 nm is determined by IR diode probing of the HCN photoproduct. The quantum yield obtained at low quencher gas pres sures, phi = 0.81 +/- 0.18, is in agreement with the value recently ob tained from molecular beam/photofragmentation studies of this process. Analysis of the quenching data within the context of the strong colli sion model allows an estimate of the first-order rate constant for HCN production from pyrazine excited at 248 nm, k(d) = 1.6 x 10(5) s(-1). Direct, IR transient absorption measurements of the HCN photoproducts confirm the mu s timescale for pyrazine dissociation extracted from t he quenching experiments. The implications of this photodissociation p rocess for the interpretation of recent collisional energy-transfer ex periments involving pyrazine and CO2 are considered. Specifically, the possibility that translationally hot HCN resulting from pyrazine diss ociation may be the source of excitation for collisions which impart a large amount of rotational and translational energy to CO2 molecules is examined. Transient absorption measurements of rotationally and tra nslationally excited CO2 molecules produced following excitation of py razine are analysed within the context of a kinetic scheme incorporati ng pyrazine photodissociation, as well as excitation of CO2 by both tr anslationally hot HCN and vibrationally excited pyrazine. This analysi s indicates that vibrationally hot pyrazine, which is above the thresh old for dissociation, is the dominant source of excitation in collisio ns which impart large amounts of rotational and translational energy t o CO2.