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The first determination of the rate of reaction of OH radicals with HBr at temperatures below 249 K is reported. Rate constants measured from 295 to 23 K increase monotonically with decrease in temperature and are faster than has previously been thought at the temperatures present in the mid and low stratosphere. The observed negative temperature dependence is well predicted by a simple formula deduced from quantum scattering calculations employing the rotating bond approximation. [ABSTRACT FROM AUTHOR]

Infrared fluorescence from HBr and C2D2(ν3) has been measured following irradiation of HBr–C2D2 mixtures with a pulsed HBr laser. The fluorescence decay from both species yields the total rate constant for deactivation of HBr(v=1) by collisions with C2D2, k=(1.6±0.1)×104 s-1 Torr-1. The exponential rise of C2D2(ν3) fluorescence is characterized by the rate constants k=(3.4±0.2)×104 s-1 Torr -1 for collisions with both C2D2 and HBr, and k=(5.5±2.3)×103 s-1 Torr-1 for collisions with added Ar. A model developed to fit the observations implies that ν3 is populated indirectly by transfer from another, unidentified, level of C2D2, and that the exponential rise constants refer to transfer from ν3 to this level. [ABSTRACT FROM AUTHOR]

An entirely new experimental method is described which enables the rate constants of neutral–neutral gas-phase reactions to be measured at ultralow temperatures. The measurements are made by applying the pulsed laser photolysis (PLP), laser-induced fluorescence (LIF) technique of studying the kinetics of free radical reactions in the ultracold environment provided by the gas flow in a Cinétique de Réaction en Ecoulement Supersonique Uniforme (CRESU) apparatus. The experimental method is described in some detail and its application and limitations are discussed. Results are reported for the reactions of CN radicals with O2 and NH3. For reaction (1) between CN and O2 data are reported for the temperature range T=13–295 K and the rate constants are well-matched by the expression k1(T)=(2.49±0.17)×10-11 (T/298)(-0.63±0.04) cm3 molecule-1 s-1. For reaction (2) between CN and NH3, rate constants in the temperature range T=25–295 K fit the expression k2(T)=(2.77±0.67)×10-11 (T/298)(-1.14±0.15) cm3 molecule-1 s-1. The kinetic data are discussed in terms of the latest quantum chemical and reaction rate theories for these systems. [ABSTRACT FROM AUTHOR]

A new method is described which enables the measurement for the first time of the rates of neutral–neutral gas-phase reactions at temperatures down to 26 K (and in the future, below). Results for the reaction of CN radicals with O2 are presented and discussed in terms of current theoretical treatments. [ABSTRACT FROM AUTHOR]