Electron Affinities of Halogen Diatomic Molecules as Determined by Endoergic Charge Transfer

Ion‐pair formation by photon absorption at threshold wavelengths has been used to prepare ${\mathrm{I}}^{-}\mathrm{,\hspace{0.17em}}{\mathrm{Br}}^{-}$ , and ${\mathrm{F}}^{-}$ ions with approximately room‐temperature thermal energies, as verified by retarding‐potential measurements. The primary ions were accelerated and their reactions with halogen molecules studied at laboratory kinetic energies from 0.0 to about 4.0 eV. Thresholds were determined for endoergic reactions of the type ${\mathrm{X}}^{-}+{\mathrm{Y}}_2\to \mathrm{X}+{\mathrm{Y}}_2^{-}$ , where X may be the same as Y. At least two reactions were used in determining each electron affinity. The agreement was good in all cases. The values of electron affinity obtained are $\text{3.08±0.1}\mathrm{eV}$ for F₂, $\text{2.38±0.1}\mathrm{eV}$ for Cl₂, $\text{2.51±0.1}\mathrm{eV}$ for Br₂, and $\text{2.58±0.1}\mathrm{eV}$ for I₂. Interhalogen molecular ions such as ${\mathrm{IBr}}^{-}$ were also observed, and measurement of the threshold for formation gave the value $\text{2.7±0.2}\mathrm{eV}$ for the electron affinity of IBr. The retarding‐potential measurements of ${\mathrm{F}}^{-}$ from F₂ strongly support a value for the dissociation energy of F₂ in the neighborhood of 1.6 eV.