Kinetic study of the gas-phase reaction of Ca(S-1(0)) with O-2 from 296 to623 K

The reactivity of gas-phase ground-state calcium atoms with O-2 is reported from 296 to 623 K, Calcium atoms were produced by the photodissociation of Ca(FOD)(2) [where FOD represents the 6,6,7.7,8,8,8-heptafluoro-2,2-dimethy l-3,5-octanedionate ion] and detected by laser-induced fluorescence. Calciu m recombines with O-2 to form CaO2 in a reaction that exhibits a positive t emperature dependence and is significantly in the falloff region even at pr essures as low as 10 Torr. Ab initio quantum calculations are used to show that the reaction most likely proceeds on a tripler surface, where the Ca a tom inserts into the O-O bond to form a triplet dioxide (B-3(2)) With a ver y low frequency bending mode. The surprisingly large rate coefficient at el evated temperatures is explained by the correspondingly high density of rov ibrational states, and the positive temperature dependence by a small barri er in the entrance channel at the crossing point between the covalent and i onic diabats. RRKM theory, fitted to the experimental data, predicts the fo llowing expression for the rate coefficient from 140 to 1000 K and 10(-5) t o 10(3) Torr: log(k(rec.0)/cm(6) molecule(-1) s(-1)) = -57.20 + 19.70 log T - 3.410(log T)(2), k(rec,infinity) = 1.36 x 10(-10) exp(-1020/T) cm(3) mol ecule(-1) s(-1), and F-c = 0.67.