Electron affinities of the alkali halides and the structure of their negative ions

Photoelectron spectra are reported for the MX (X̃ ¹∑ ⁺) + e^-←MX^-(X̃ ²∑ ⁺) transitions often alkali halide anions at 488 nm. Adiabatic electron affinities (±0.010 eV) are determined to be 0.593 (LiCl), 0.520 (NaF), 0.727 (NaCl), 0.788 (NaBr), 0.865 (NaI), 0.582 (KCl), 0.642 (KBr), 0.728 (KI), 0.543 (RbCl), and 0.455 eV (CsCl). Fundamental vibrational frequencies, equilibrium bond lengths, and dissociation energies are also reported for the anion 2Σ⁺ ground states. An observed linear correlation of electron affinities with α/r² (α = metal atom polarizability) is used to predict the electron affinities of the remaining alkali bromides and iodides, as well as related alkali salts. A simple electrostatic model for the alkali halide anions is also presented which enables the accurate (±0.1 eV) calculation of electron affinities.