Photoelectron Imaging Spectroscopy of Ta 4 O n − ( n = 0–9) Clusters

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.