Kinetic studies of the reactions of OH(X2Π) with hydrogen chloride and deuterium chloride at elevated temperatures by time-resolved resonance fluorescence (A2∑+–X2Π)

Abstract

We present a kinetic study of the reactions of the OH radical with the molecules HCl and DCl over the temperature range 300–700 K in order to investigate the ‘intermediate’ region between room-temperature measurements and flame investigations. Ground-state OH radicals were generated by the repetitive pulsed irradiation of H2O vapour in a flow system, kinetically equivalent to a static system, and monitored in the time-resolved mode by resonance fluorescence at λ= 307 nm [OH(A2∑+→X2Π), (0, 0)] following optical excitation. Decay profiles of the OH radical in the presence of HCl and DCl were constructed following pre-trigger photomultiplier gating, photon counting, signal averaging and computerised analysis. A specially constructed high-temperature stainless-steel reactor, particularly designed for photon-counting measurements on a high-temperature system and critical to the extension of previous rate measurements of such reaction of OH by ca. 250 K, is described in detail with particular emphasis for procedures for minimising large background photon counts normally encountered in this type of system. The resulting rate data obtained in this investigation for the range 300–700 K may be summarised in the Arrhenius forms: OH (X2Π)+ HCl: kR=(2.94 ± 0.48)× 10–12 exp[–(446 ± 32)/T] cm3 molecule–1 s–1, OH (X2Π)+ DCl: kR=(4.04 ± 0.74)× 10–12 exp[–(718 ± 33)/T) cm3 molecule–1 s–1.