Photoelectron Imaging Spectroscopy of Ta ₄ O _n ⁻ (

The structural and electronic properties of oxygen‐deficient Ta ₄ O _n ⁻ ( n  = 0–9) clusters were investigated using cryogenic photoelectron imaging spectroscopy in combination with density functional theory calculations. The electron affinities of neutral Ta ₄ O _n ( n  = 0–4) are measured to be 1.107 ± 0.005, 1.340 ± 0.005, 1.302 ± 0.005, 1.485 ± 0.005 and 1.419 ± 0.005 eV, respectively. The symmetric Ta–Ta stretching frequencies are measured as 169 cm ⁻¹ for Ta ₄ O ₂ ⁻ and 145 cm ⁻¹ for Ta ₄ O ₄ ⁻ , while an asymmetric Ta–Ta stretching at 129 cm ⁻¹ was identified for Ta ₄ O ₃ ⁻ . Structural analysis reveals oxygen atoms preferentially occupy bridging sites on the tetrahedral Ta ₄ framework core, with terminal coordination becoming favorable only at relatively high oxygen content. A pronounced odd–even alternation in the adiabatic detachment energies was observed for n  = 1–4. Orbital delocalization index analysis reveals that this oscillation originates from variations in the localization of the highest occupied molecular orbital, leading to stronger electron binding in odd‐ n clusters. Benchmarking against high‐quality spectroscopic data shows that the BP86 functional yields a robust description of the electronic structure of these clusters, whereas commonly employed hybrid functionals show significant deviations.