Photoelectron Imaging Spectroscopy of Ta₄O_n ^- (n = 0-9) Clusters

The structural and electronic properties of oxygen-deficient Ta₄O_n ^- (n = 0-9) clusters were investigated using cryogenic photoelectron imaging spectroscopy in combination with density functional theory calculations. The electron affinities of neutral Ta₄O_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 Ta₄O₂ ^- and 145 cm^-1 for Ta₄O₄ ^-, while an asymmetric Ta-Ta stretching at 129 cm^-1 was identified for Ta₄O₃ ^-. Structural analysis reveals oxygen atoms preferentially occupy bridging sites on the tetrahedral Ta₄ 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.