Quantum yields and primary processes in the photolysis of HNCO vapor at 2062 Å

Abstract

Quantum yields of CO, N2, and H2 have been measured in the photolysis of HNCO vapor (2–23 Torr, 25 °C) at 2062 Å. Extrapolated values at zero pressure of HNCO were about 1.0, 0.40, and 0.13 respectively. Φ(CO) and Φ(N2) increased with increasing HNCO pressure and became approximately constant at values of 1.5 and 0.6 respectively between 10 and 20 Torr. From these and other data it was concluded that two primary processes, each with a quantum yield of about 0.5, occur.[Formula: see text]The abstraction of H from HNCO by methyl radicals was found to be slow, even at 250 °C, and it was concluded that abstraction of H from HNCO plays no part in the photolysis mechanism, contrary to previous conclusions. A mechanism is suggested, based on radical–radical reactions at low HNCO pressure or high intensity, with radical reactions with HNCO becoming important at higher pressures or low intensity. It is concluded that H2 was formed in the system by reactions of H atoms.