An ion-molecule mechanism for the formation of neutral sporadic Na layers

This paper describes a laboratory study into the chemical pathways by which Na+ is converted to Na in the upper atmosphere. The termolecular clustering reactions of Na+ with N-2, O-2, and CO2 were studied in a low-temperature fast flow reactor coupled to a quadrupole mass spectrometer. This yielded k(Na+ + N-2 + He, 93-255 K) = (1.20 +/- 0.13) x 10(-30) (T/200 K)(-(2.20+/-0.09)), k(Na+ + O-2 + He, 93-130 K) = (5.20 +/- 2.62) x 10(-31) (T/200 K)(-(2.64+/-0.74)), k(Na+ + CO2 + He, 158-300 K) = (9.05 +/- 1.38) x 10(-30) (T/200 K)(-(2.84+/-0.48)), where the units are cm(6) molecule(-2) s(-1) and the stated errors are a combination of the 2 sigma standard errors in the kinetic data and the systematic errors in the temperature, pressure, and flour rates. It was then shown that atomic O will ligand switch with Na.N-2(+) but not with. Na.CO2+, and that the former reaction proceeds essentially at the Langevin collision frequency. The neutralization of Na+ in the upper atmosphere is therefore rather complex. The first step is formation of the Na.N-2(+) ion from the recombination of Na+ with N-2. This cluster ion can then either switch with CO2, which leads to a stable cluster ion that will undergo dissociative electron recombination to form Na; or switch with atomic O, which reforms Na+. The result of this is that the lifetime of Na+ changes very rapidly from more than a day above 100 km to just a few minutes at 90 km. Furthermore, the rate of neutralization is largely independent of the electron concentration A simple model describing the conversion of Na+ to atomic Na in a descending sporadic E layer demonstrates that this ion-molecule mechanism appears to fulfil many of the major criteria for producing sporadic sodium layers.