O2(a1Δg)+Mg, Fe, and Ca: Experimental kinetics and formulation of a weak collision, multiwell master equation with spin-hopping

The first excited electronic state of molecular oxygen, O-2(a(1)Delta(g)), is formed in the upper atmosphere by the photolysis of O-3. Its lifetime is over 70 min above 75 km, so that during the day its concentration is about 30 times greater than that of O-3. In order to explore its potential reactivity with atmospheric constituents produced by meteoric ablation, the reactions of Mg, Fe, and Ca with O-2(a) were studied in a fast flow tube, where the metal atoms were produced either by thermal evaporation (Ca and Mg) or by pulsed laser ablation of a metal target (Fe), and detected by laser induced fluorescence spectroscopy. O-2(a) was produced by bubbling a flow of Cl-2 through chilled alkaline H2O2, and its absolute concentration determined from its optical emission at 1270 nm (O-2(a(1)Delta(g) -X-3 Sigma(-)(g)). The following results were obtained at 296 K: k(Mg + O-2(a) + N-2 -> MgO2 + N-2) = (1.8 +/- 0.2) x 10(-30) cm(6) molecule(-2) s(-1); k(Fe + O-2(a) -> FeO + O) = (1.1 +/- 0.1) x 10(-13) cm(3) molecule(-1) s(-1); k(Ca + O-2(a) + N-2 -> CaO2 + N-2) = (2.9 +/- 0.2) x 10(-28) cm(6) molecule(-2) s(-1); and k(Ca + O-2(a) -> CaO + O) = (2.7 +/- 1.0) x 10(-12) cm(3) molecule(-1) s(-1). The total uncertainty in these rate coefficients, which mostly arises from the systematic uncertainty in the O-2(a) concentration, is estimated to be +/- 40%. Mg + O-2(a) occurs exclusively by association on the singlet surface, producing MgO2((1)A(1)), with a pressure dependent rate coefficient. Fe + O-2(a), on the other hand, shows pressure independent kinetics. FeO + O is produced with a probability of only similar to 0.1%. There is no evidence for an association complex, suggesting that this reaction proceeds mostly by nearresonant electronic energy transfer to Fe(a(5)F) + O-2(X). The reaction of Ca + O-2(a) occurs in an intermediate regime with two competing pressure dependent channels: (1) a recombination to produce CaO2((1)A(1)), and (2) a singlet/triplet non-adiabatic hopping channel leading to CaO + O(P-3). In order to interpret the Ca + O-2(a) results, we utilized density functional theory along with multireference and explicitly correlated CCSD(T)-F12 electronic structure calculations to examine the lowest lying singlet and triplet surfaces. In addition to mapping stationary points, we used a genetic algorithm to locate minimum energy crossing points between the two surfaces. Simulations of the Ca + O-2(a) kinetics were then carried out using a combination of both standard and non-adiabatic Rice-Ramsperger-Kassel-Marcus (RRKM) theory implemented within a weak collision, multiwell master equation model. In terms of atmospheric significance, only in the case of Ca does reaction with O-2(a) compete with O-3 during the daytime between 85 and 110 km. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4730423]