Competition between photochemistry and energy transfer in ultraviolet-excited diazabenzenes. II. Identifying the dominant energy donor for "supercollisions"

CO2 bath molecules scattered into J = 72 of the 00(0)0 vibrational state at short times after 248 or 266 nm UV excitation of pyrazine are probed using high resolution time resolved IR diode laser spectroscopy as a function of UV laser fluence from similar to 3 to 80 mJ/cm(2). The implications of pyr azine photodissociation for the interpretation of these collisional energy transfer experiments are considered. Specifically, the possibility that tra nslationally hot HCN resulting from pyrazine dissociation may be the source of excitation for collisions that impart a large amount of rotational and translational energy to CO2 molecules is examined. Transient absorption mea surements probing rotationally and translationally excited CO2 molecules pr oduced following excitation of pyrazine are analyzed within the context of a kinetic scheme incorporating pyrazine photodissociation, as well as excit ation of CO2 by both translationally hot HCN and vibrationally excited pyra zine. This analysis indicates that vibrationally hot pyrazine, which has su fficient energy to dissociate, is the source of excitation in collisions im parting large amounts of rotational and translational energy to CO2. (C) 20 00 American Institute of Physics. [S0021-9606(00)00410-4].