Measurement of electron affinity of iridium atom and photoelectron angular distributions of iridium anion

The latest electron affinity value of an iridium atom is 1.564 36(15) eV, determined via a method based on the Wigner threshold law by Bilodeau and co-workers. However, they observed a significant deviation from the Wigner threshold law in the threshold photodetachment experiment. To address this dilemma, we conducted high-resolution photoelectron spectroscopy of Ir^- via the slow-electron velocity-map imaging method in combination with an ion trap. The electron affinity of Ir was measured to be 12 614.97(9) cm^-1 or 1.564 057(11) eV. We find that the Wigner threshold law is still valid for the threshold photodetachment of Ir^- through a p-wave fitting of the photodetachment channel Ir^-5d⁸6s²³F₄→Ir5d⁸6sb⁴F_9/2. The photoelectron angular distributions of photodetachment channels Ir^-5d⁸6s²³F₄→Ir5d⁷6s²a⁴F_9/2 and Ir^-5d⁸6s²³F₄→Ir5d⁸6sb⁴F_9/2 were also investigated. The behavior of anisotropy parameter β indicates a strong interaction between the two channels. Moreover, the energy level ³P₂ of Ir^-, which was not observed in the previous works, was experimentally determined to be 4163.24(16) cm^-1 above the ground state.