The formation of negative ions by electron capture in gases in which a dissociation process does not occur is explained by a unimolecular process involving the excitation of molecular vibrational levels and subsequent loss of energy by collision or resonance. In order to obtain a proper order of magnitude to agree with experimental observations, one must assume a change of only one vibrational quantum number. This sets an upper limit on the electron affinity. For the case of ${\mathrm{O}}_2$, this limit is 0.17 volt consistent with other observations. The theory also yields a dependence of the phenomenon on the average energy of the electrons which is in agreement with experiment.

• Received 19 June 1935

DOI:https://doi.org/10.1103/PhysRev.48.689