A KINETIC INVESTIGATION OF THE CA/CAO SYSTEM - NON-ARRHENIUS BEHAVIOR OF THE REACTION CA(S-1) + N2O OVER THE TEMPERATURE-RANGE 250-898-K AND A STUDY OF THE REACTION CAO + O

A study is presented of the reaction Ca(1S) + N2O over the temperature range 250-898 K. Ca atoms were produced in an excess of N2O and He bath gas by the pulsed two-photon (193.3 nm) dissociation of CaI2 and then monitored by time-resolved laser-induced fluorescence spectroscopy at λ = 422.7 nm (Ca(41P1-41S)). Above 500 K there is a clear upward curvature in the Arrhenius plot, and the best description of the temperature dependence of the rate constant over the experimental temperature range is given by k(T) = (2.43 ± 0.29) × 10-11 exp[-(6.84 ± 0.27) kJ mol-1/RT] + (9.70 ± 1.39) × 10-10 exp[-(24.58 ± 1.09) kJ mol-1/RT] cm3 molecule-1 s-1. Such behavior is explained by vibrationally excited N2O enhancing the reaction at high temperatures. This rate coefficient expression, and the chemiluminescence cross section obtained in a previous study of the title reaction in a beam-scattering experiment, are used to estimate a photon yield of 12.5%. This yield is shown to be about half that predicted by a statistical consideration of the densities of states in the reactant electronic channels. In a separate experiment, the two-photon (193.3 nm) dissociation of CaO, produced by mixing a flow of Ca atoms and N2O, is used to study the reaction CaO + O → Ca + O2, which proceeds at (6.5 ± 2.2) × 10-10 cm3 molecule-1 s-1 at 805 K. © 1990 American Chemical Society.