Pressure and temperature dependence of the rate constants for the association reaction of OH radicals with NO between 301 and 23 K

Abstract

The pulsed laser photolysis (PLP), time-resolved laser-induced fluorescence (LIF) technique has been used to study the association reaction OH + NO(+M)→ HONO(+M) at low and ultra-low temperatures. Using a cryogenically cooled cell, rate constants have been determined at temperatures down to 80 K and with M = N₂ over a range of total gas densities. The PLP–LIF method has also been implemented in the ultra-cold environment provided by the gas flow in a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus yielding rate constants at temperatures as low as 23 K. At 52 K, rate constants have been measured at total gas densities from 5.1 × 10¹⁶ to 8.2 × 10¹⁷ molecule cm^–3. The results reported here are the first for any neutral–neutral radical recombination at such low temperatures. The pressure and temperature dependence of the rate constants observed in the cryogenically cooled cell are fitted to obtain rate constants in the limits of low and high pressure given by the expressions: K°(T)[N₂]= 8.9 × 10^–31(T/298)^–2.1[N₂] cm³ molecule^–1 s^–1K^∞(T)≈ 5.4 × 10^–11(T/298)^–0.1 cm³ molecule^–1 s^–1

The rate constants determined at ultra-low temperatures in the CRESU apparatus are used to derive values of limiting low-pressure rate constants for the combinations of temperature and third-body gas (Ar, N₂ and He) employed in these experiments. The rate constants for M = Ar at temperatures between 295 and 23 K are fitted well by the following expression for the limiting low-pressure rate constant: K°(T)[Ar]= 4.5 × 10^–31(T/298)^–2.6[Ar] cm³ molecule^–1 s^–1

The limiting low-pressure rate constants deduced from the experiments are compared with values calculated using the methodology of Troe (J. Chem. Phys., 1977, 66, 4758). The results are in fair agreement.