New studies of the unimolecular reaction NO2O+NO. Part 1. High pressure range of the O+NO recombination between 200 and 400 K

Abstract

The falloff curve of the recombination O+NO+N₂→NO₂+N₂ has been studied at temperatures between 200 and 400 K and at pressures between 2 and 200 bar. Oxygen atoms were produced by laser flash photolysis of N₂O at 193 nm, NO₂ was detected by light absorption at 405 nm. The results supersede earlier data from relative rate measurements. Neglecting the effects of an onset of diffusion control at high pressures, limiting rate constants k_rec,∞ of 4.7×10^-11 and 4.0×10^-11 cm³ molecule^-1 s^-1 at 200 and 300 K, respectively, are obtained by extrapolation. Accounting for diffusion control increases the extrapolated values by 30 to 50% such that, for example, k_rec,∞(300 K)≈5±1×10^-11 cm³ molecule^-1 s^-1. Rate constants for the high pressure limiting rate constants for recombination and dissociation, as well as for the vibrational relaxation of NO by oxygen atoms, over the range 200–2700 K are compared. Most of the observed temperature dependence of k_∞ can be attributed to that of the thermal population of the lowest electronic fine structure components of O and NO. The remaining temperature dependence is estimated to be weaker than T^-0.5.