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 I−, Br−, and 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 X−+Y2→X+Y2 −, 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 3.08±0.1 eV for F2, 2.38±0.1 eV for Cl2, 2.51±0.1 eV for Br2, and 2.58±0.1 eV for I2. Interhalogen molecular ions such as IBr− were also observed, and measurement of the threshold for formation gave the value 2.7±0.2 eV for the electron affinity of IBr. The retarding-potential measurements of F− from F2 strongly support a value for the dissociation energy of F2 in the neighborhood of 1.6 eV.