The Heat of Dissociation of CO and the Electron Affinity of O

Abstract

An apparatus previously described has been employed in a study of the dissociation produced by primary electron impact in CO and ${\mathrm{O}}_2$. The processes studied in ${\mathrm{O}}_2$ are ${\mathrm{O}}_2^{-}$→${\mathrm{O}}^{-}$+O at 2.9 v and ${\mathrm{O}}_2^{-}$→${\mathrm{O}}^{-}$+O* at 12.0 v with an estimated accuracy of ±0.2 v. These figures refer to the potential energy of the dissociation products, the normal molecule considered as zero. These results give two values of $E_0$, the electron affinity of the oxygen atom, both equal to 2.2±0.2 v. The processes studied in CO are C${\mathrm{O}}^{-}$→C+${\mathrm{O}}^{-}$ at 9.5 v, CO→${\mathrm{C}}^{+}$+${\mathrm{O}}^{-}$ at 20.9 v, and C${\mathrm{O}}^{+}$→${\mathrm{C}}^{+}$+O at 22.8 v, with an estimated accuracy of ±0.1 v. It is concluded that $D\left(\mathrm{CO}\right)$ may have one of two possible values, 9.6±0.1 or 11.6±0.1 v. These experiments are unable to decide between the two but most of the evidence from other sources points to the lower value. If $D\left(\mathrm{CO}\right)=9.6\mathrm{}$ v we must admit the possibility of stable excited ${\mathrm{O}}^{-}$ ions. However, if $D\left(\mathrm{CO}\right)=11.6\mathrm{}$ v, these experiments on CO give two values of $E_0$, 1.9 and 2.1 v. The processes involved in the formation of negative ions by electron impact are discussed.