Gas phase studies of Na diffusion in He and Ar and kinetics of Na+Cl2and Na+SF6

Reaction Kinetics of Meteoric Sodium Reservoirs in the Upper Atmosphere

The gas-phase reactions of a selection of sodium-containing species with atmospheric constituents, relevant to the chemistry of meteor-ablated Na in the upper atmosphere, were studied in a fast flow tube using multiphoton ionization time-of-flight mass spectrometry. For the first time, unambiguous observations of NaO and NaOH in the gas phase under atmospheric conditions have been achieved. This enabled the direct measurement of the rate constants for the reactions of NaO with H2, H2O, and CO, and of NaOH with CO2, which at 300-310 K were found to be (at 2σ confidence level): k(NaO + H2O) = (2.4 ± 0.6) × 10(-10) cm(3) molecule (-1) s(-1), k(NaO + H2) = (4.9 ± 1.2) × 10(-12) cm(3) molecule (-1) s(-1), k(NaO + CO) = (9 ± 4) × 10(-11) cm(3) molecule (-1) s(-1), and k(NaOH + CO2 + M) = (7.6 ± 1.6) × 10(-29) cm(6) molecule (-2) s(-1) (P = 1-4 Torr). The NaO + H2 reaction was found to make NaOH with a branching ratio ≥ 99%. A combination of quantum chemistry and statistical rate theory calculations are used to interpret the reaction kinetics and extrapolate the atmospherically relevant experimental results to mesospheric temperatures and pressures. The NaO + H2O and NaOH + CO2 reactions act sequentially to provide the major atmospheric sink of meteoric Na and therefore have a significant impact on the underside of the Na layer in the terrestrial mesosphere: the newly determined rate constants shift the modeled peak to about 93 km, i.e., 2 km higher than observed by ground-based lidars. This highlights further uncertainties in the Na chemistry cycle such as the unknown rate constant of the NaOH + H reaction. The fast Na-recycling reaction between NaO and CO and a re-evaluated rate constant of the NaO + CO2 sink should be now considered in chemical models of the Martian Na layer.

Generation of iron atoms in the gas phase by microwave-induced plasma afterglows

A fast-flow reactor technique is described by which Fe atoms can be produced in the gas phase in the afterglow of microwave-induced plasmas in hydrogen/argon and hydrogen/helium mixtures. When the iron salt FeCl3(s) was brought into the gas phase by thermal sublimation at temperatures between 360 and 405 K, it was partly converted to Fe atoms by reaction of the gaseous compounds FexCl3x(g) with hydrogen atoms. The Fe atoms were detected by atomic absorption spectroscopy (AAS). It was shown that sublimation of the salt is the rate-determining step of the overall plasma-afterglow atomisation process. Experimental conditions for the generation of Fe atoms suited to kinetic studies start at a temperature of 303 K. In the downstream region the concentration of Fe atoms decays due to diffusion to the reactor wall. Binary diffusion coefficients DFe/Ar and DFe/He of 231.5 +/- 6.6 and 370.0 +/- 15.5 cm2 s-1 Torr at 303 K, respectively, were determined.