Radical-molecule reaction dynamics in the low temperature regime.

Abstract

The temperature dependence of the rate coefficient of radical-molecule reactions is studied using novel flow techniques. The methodology makes use of the cooling extant in uniform and free supersonic expansions to produce low temperature (90-250 K in the uniform expansion and near 1 K in the free jet) environments which are not at chemical equilibrium. The design of the axisymmetric Laval nozzles, which are integral in producing the uniform supersonic expansion, is reviewed in depth. The experimental details pertinent to the characterization and operation of the pulsed uniform supersonic expansion flow reactor are considered. The application of free jet flows for the study of radical-molecule reactions at temperatures near I K is also discussed. The rate coefficient of the atmospherically important termolecular reactions OH + NO and OH + NO₂ were investigated at several temperatures between approximately 100 and 250 K and pressures between 0.1 and 10. The results are found to be well fit by a simple k = CT⁻ⁿ dependence suggested by statistical unimolecular theory within the collision complex model. The suggested variation of the reaction, OH + NO, is k = 7.0(±2.0) X 10⁻³¹ (T/300 K)^(2.6±0.3) over the range 90 to 550 K. The reaction, OH + N02, displayed falloff behavior in the pressure range studied here, so recommendation of the absolute low pressure rate coefficient awaits further pressure dependent study. The purely bimolecular reaction, OH + HBr, has been studied at temperatures between 76 and 242 K using the pulsed uniform supersonic expansion flow reactor and near 1 K using the free jet flow reactor. This reaction displays a complex temperature dependence not well predicted by current dynamical theory. The results are evaluated in the light of previously obtained rate coefficients, generally at higher temperatures, and of theoretical predictions of the absolute value and temperature dependence of the rate coefficient. Impact on the modeling of low temperature environments is discussed.