ULTRALOW TEMPERATURE KINETICS OF NEUTRAL-NEUTRAL REACTIONS - THE TECHNIQUE AND RESULTS FOR THE REACTIONS CN-2 DOWN TO 13 K AND CN+NH3 DOWN TO 25 K(O)

An entirely new experimental method is described which enables the rat e constants of neutral-neutral gas-phase reactions to be measured at u ltralow 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-envi ronment provided by the gas flow in a Cinetique de Reaction en Ecoulem ent Supersonique Uniforme (CRESU) apparatus. The experimental method i s described in some detail and its application and limitations are dis cussed. Results are reported for the reactions of CN radicals with O-2 and NH3. For reaction (1) between CN and O-2 data are reported for th e temperature range T=13-295 K and the rate constants are well-matched by the expression k(1)(T)=(2.49+/-0.17)X10(-11) (T/298)((-0.63+/-0.04 )) cm(3) molecule(-1) s(-1). For reaction (2) between CN and NH3, rate constants cm in the temperature range T=25-295 K fit the expression k (2)(T)=(2.77+/-0.67)X 10(-11) (T/298)((-1.14+/-0.15)) Cm-3 molecule(-1 ) s(-1). The kinetic data are discussed in terms of the latest quantum chemical and reaction rate theories for these systems.