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.

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. The rate coefficient of the reaction Si( 3 PJ ) + O2 → SiO + O has been measured at temperatures down to 15 K using a CRESU (Cinétique de Réaction enÉcoulement Supersonique Uniforme) apparatus coupled with the PLP-LIF (Pulsed Laser Photolysis -Laser Induced Fluorescence) technique. The temperature dependence of the rate coefficient is well fitted using the expression: 1.72×10−10 (T /300 K) −0.53 exp(−17 K/T ) cm 3 molecule −1 s −1 in the temperature range 15-300 K. The silicon chemistry in interstellar clouds is reviewed and possible consequences of our study are stressed.

The first direct measurement of the reaction rate constant of a polycyclic aromatic hydrocarbon in the gas phase in the temperature range 58-470 K is reported. The reaction is OH+ anthracene and the experiment has been performed in a continuous flow Cinetique de Reaction en Ecoulement Supersonique Uniforme apparatus, which had to be modified for this purpose. Pulsed laser photolysis of H(2)O(2) has been used to generate OH radicals and laser-induced fluorescence to observe the kinetic decay of the radicals and hence determine the rate coefficients. The reaction is found to be fast, and the rate constant increases monotonically as the temperature is lowered. The rate coefficients match the expression k(cm(3) molecules(-1) s(-1))=1.12 x 10(-10)(T/300)(-0.46).