Threshold Shapes and Resonances in the Photodetachment Cross Sections of Chloride, Bromide, and Iodide

Abstract

The photodetachment curves of Cl^—, Br^—, and I^— have been examined in their threshold regions by absorption spectroscopy of shock‐heated alkali halide vapors. The chloride curve appears smooth and normal in terms of threshold laws, but the bromide and iodide curves show maxima about 500 cm⁻¹ above the thresholds, with widths of about 200 cm⁻¹ at half‐height. The threshold positions are not affected by changes in conditions. The anomalously large absorption or resonances seem to be due to the halide ions, but to be influenced to some degree by the alkali atoms or ions. Three explanations, none entirely satisfactory, are put forth for the maxima; these are (1) near‐resonant transfer of energy from X+e^— to M, giving X^—+M^*; (2) metastable autodetaching states of the halides; and (3) mixing of vacant alkali ion orbitals with continua of the halides. The third is considered most plausible.