Kinetics of the Radical−Radical Reaction, O(³P_J) + OH(X²Π_Ω) → O₂ + H, at Temperatures down to 39 K

Abstract

The kinetics of the reaction between O atoms and OH radicals, both in their electronic ground state, have been investigated at temperatures down to ca. 39 K. The experiments employed a CRESU (Cinétique deRéaction en Ecoulement Supersonique Uniforme) apparatus to attain low temperatures. Both reagents were created using pulsed laser photolysis at 157.6 nm of mixtures containing H2O and O2 diluted in N2 carrier gas. OH radicals were formed by both direct photolysis of H2O and the reaction between O(1D) atoms and H2O. O(3P) atoms were formed both as a direct product of O2 photolysis and by the rapid quenching of O(1D) atoms formed in that photolysis by N2 and O2. The rates of removal of OH radicals were observed by laser-induced fluorescence, and concentrations of O atoms were estimated from a knowledge of the absorption cross-section for O2 at 157.6 nm and of the measured fluence from the F2 laser at this wavelength. To obtain a best estimate of the rate constants for the O + OH reaction, we had to correct the raw experimental data for the following: (a) the decrease in the laser fluence along the jet due to the absorption by O2 in the gas mixture, (b) the increase in temperature, and consequent decrease in gas density, as a result of energy released in the photochemical and chemical processes that occurred, and (c) the formation of OH(v = 0) as a result of relaxation, particularly by O2, of OH radicals formed in levels v > 0. Once these corrections were made, the rate constant for reaction between OH and O(3P) atoms showed little variation in the temperature range of 142 to 39 K and had a value of (3.5 +/- 1.0) x 10(-11) cm3 molecule(-1) s(-1). It is recommended that this value is used in future chemical models of dense interstellar clouds.

Formation of molecular oxygen in ultracoldO+OHcollisions

“…They showed that magnetic fields of several thousand Gauss suppress inelastic collision rates by about two orders of magnitude. In a recent study [33], we reported quantum dynamics calculations of reaction (1) for T = 10 − 600 K and found no significant decrease of the rate coefficient in the temperature range 39 − 10 K, in accordance with conclusions of a recent experimental study by Carty et al [52]. Our calculations for reaction probabilities were in excellent agreement with those of Xu et al [53] for collision energies E c > 0.012 eV.…”

“…[33] and experimental conclusions of Ref. [52], that the rate coefficient of reaction (1) is unlikely to vanish for T < 10 K. We also note that the Langevin rate coefficient is in semiquantitative agreement with the inelastic rate coefficient in the range T = 100−1000 K. The difference is less than 30 %.…”

Oxygen depletion in dense molecular clouds: a clue to a low O2abundance?

“…The experimental rate constant is well determined between 140 and 300 K decreasing from 7 × 10 −11 cm 3 molecule −1 s −1 at 140 K to 3 × 10 −11 cm 3 molecule −1 s −1 at 300 K. Between 40 and 140 K, the reaction has been studied in a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus (Carty et al 2006) leading to a value of around 3.5(±1.0) × 10 −11 cm 3 molecule −1 s −1 which however has large uncertainties. Quasi-classical trajectory calculations give good agreement with experiment between 300 K and 3000 K (Troe & Ushakov 2001) and between 40 K and 140 K (Jorfi et al 2008) but the relatively good agreement at low temperature may be fortuitous.…”

$O^{18}$O and $C^{18}$O observations ofρOphiuchi A

“…The thermal rate coefficient of this reaction has recently been measured in the laboratory down to temperatures of 39 K (Carty et al 2006), where it remains rapid. In subsequent ab initio theoretical calculations, Xu et al (2007) found a much smaller rate at temperatures below 30 K and suggested that this might solve the problem of missing O 2 in cold interstellar clouds.…”