Electron affinity of tantalum and excited states of its anion.

The tantalum anion has the most complicated photoelectron spectrum among all atomic anions of transition elements, which was the main obstacle to accurately measure its electron affinity via the generic method. The latest experimental value of the electron affinity of Ta was 0.323(12) eV, reported by Feigerle et al. [J. Chem. Phys. 74, 1580 (1981)]. In the present work, we report the high-resolution photoelectron spectroscopy of Ta via the slow-electron velocity-map imaging method combined with a cryogenic ion trap. The electron affinity of Ta was measured to be 2652.38(17) cm or 0.328 859(23) eV. Three excited states D, P, and D of Ta were observed, and their energy levels were determined to be 1169.64(17) cm for D, 1735.9(10) cm for P, and 2320.1(20) cm for D above the ground state D, respectively.