A study of the dissociative recombination of CaO(+) with electrons: Implications for Ca chemistry in the upper atmosphere

The dissociative recombination of CaO(+) ions with electrons has been studied in a flowing afterglow reactor. CaO(+) was generated by the pulsed laser ablation of a Ca target, followed by entrainment in an Ar(+) ion/electron plasma. A kinetic model describing the gas‐phase chemistry and diffusion to the reactor walls was fitted to the experimental data, yielding a rate coefficient of (3.0 ± 1.0) × 10(−7) cm(3) molecule(−1) s(−1) at 295 K. This result has two atmospheric implications. First, the surprising observation that the Ca(+)/Fe(+) ratio is ~8 times larger than Ca/Fe between 90 and 100 km in the atmosphere can now be explained quantitatively by the known ion‐molecule chemistry of these two metals. Second, the rate of neutralization of Ca(+) ions in a descending sporadic E layer is fast enough to explain the often explosive growth of sporadic neutral Ca layers.