Near-threshold laser spectroscopy of iridium and platinum negative ions: Electron affinities and the threshold law

The electron affinity of Ir is measured to be 12617.4(12) cm^-1 [1.56436(15) eV], from photodetachment studies on Ir^-. Previous measurements of the electron affinity of platinum reported results which were inconsistent within quoted error bars. A photodetachment study with a very improved energy resolution, signal-to-noise ratio, and signal-to-background ratio, was conducted on Pt^-, and yields a much more accurate electron affinity for Pt of 17140.1(4) cm^-1 [2.12510(5) eV], in good agreement with the most recent measurement. Possible explanations for the poor agreement between the earlier results are discussed. In both the Ir^- and Pt^- spectra, the data indicate that the detachment cross section deviates from the expected Wigner threshold law, even near the detachment threshold. This behavior cannot be explained by the correction terms to the Wigner law proposed by the currently available threshold detachment models.