Ultralow temperature kinetics of neutral–neutral reactions. The technique and results for the reactions CN+O2 down to 13 K and CN+NH3 down to 25 K

Abstract

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.