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.

The first experiments on the kinetics of reactions of the OH radical at temperatures below 80 K are reported. They have been carried out by applying t h e pulsed laser photolysis (PLP), laser-induced fluorescence (LIF) technique for studying the kinetics of free radical reactions in t h e ultracold environment provided by the gas flow in a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus. This method has yielded rate constants for the reactions of OH with but-1-ene, (Z)-but-2-ene and (€)-but-2-ene at temperatures down to 23 K. The rate constants for all three reactions increase monotonically as the temperature is lowered and this dependence of the rate constants on temperature can be fitted to an empirical expression of t h e form (k/tO-" cm3 molecule-' s-') = a exp[b(T/298K)] with a and b equal to 5.2 and -2.8 for but-1-ene, 4.7 and -2.1 for (Z)-but-2-ene and 5.4 and -2.1 for (€)-but-2-ene.

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

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Nous présentons une méthode de mesure indirecte de la diffusion inélastique d'électrons sous grands angles. Cette méthode est basée sur la mesure du recul d'atomes d'un faisceau supersonique excités par choc électronique et détectés dans des états métastables. Nous appliquons cette méthode à l'étude des résonances de l'hélium. Nous montrons que l'état résonnant He-***(2 s2 2p) 2P se manifeste par une augmentation de la section efficace de rétrodiffusion inélastique des électrons