Supercollisions, photofragmentation and energy transfer in mixtures of pyrazine and carbon dioxide

Abstract

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 pressures, ϕ= 0.81 ± 0.18, is in agreement with the value recently obtained from molecular beam/photofragmentation studies of this process. Analysis of the quenching data within the context of the strong collision model allows an estimate of the first-order rate constant for HCN production from pyrazine excited at 248 nm, k_d= 1.6 × 10⁵ s^–1. Direct, IR transient absorption measurements of the HCN photoproducts confirm the µs timescale for pyrazine dissociation extracted from the quenching experiments. The implications of this photodissociation process for the interpretation of recent collisional energy-transfer experiments involving pyrazine and CO₂ are considered. Specifically, the possibility that translationally hot HCN resulting from pyrazine dissociation 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 translationally excited CO₂ molecules produced following excitation of pyrazine are analysed within the context of a kinetic scheme incorporating pyrazine photodissociation, as well as excitation of CO₂ by both translationally hot HCN and vibrationally excited pyrazine. This analysis indicates that vibrationally hot pyrazine, which is above the threshold for dissociation, is the dominant source of excitation in collisions which impart large amounts of rotational and translational energy to CO₂.