Abstract

The relatively long-lived afterglows which are observed downstream from electric discharges through flowing gases are due to emission by electronically excited molecules formed by the recombination of free atoms produced in the discharge. Most afterglows arise by the recombination of atoms in their electronic ground states (e.g. N+N, N+O, O+O, S+S, Cl+Cl, etc.) or by their combination with diatomic molecules, as in the air afterglow from O+NO or the carbon dioxide afterglow from O+CO. Two mechanisms obtain: two-body combination which is the inverse of predissociation in molecular spectra, and combination into an excited electronic state in the presence of a third body. The three-body process normally predominates and often gives high yields of emitting states; radiationless processes have considerable importance since the emitting states do not normally correlate with the atomic states from which they are formed. In the inert gas afterglows, the emitting state arises by the combination of a metastable excited atom with a ground state atom. In this field, there is generally good agreement between theory and experiment, and the study of afterglows can provide much information about the behaviour of excited states of molecules.

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