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 CO 2 were studied in a low-temperature fast flow reactor coupled to a quadrupole mass spectrometer. This yielded κ(Na + + N 2 + He, 93-255 K) = (1.20 ± 0.13) x 10 -30 (T/200 K) -(2.20±0.09) , κ(Na + + O 2 + He, 93-130 K) = (5.20 ± 2.62) x 10-31 (T/200 K) -(2.64±0.74) κ(Na + + CO 2 + 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 2σ standard errors in the kinetic data and the systematic errors in the temperature, pressure, and flow rates. It was then shown that atomic O will ligand switch with Na.N 2 + but not with Na.CO 2 + , 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 CO 2 , 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.