Electronic Structures of Small Stoichiometric ZnO Clusters

Stoichiometric ZnO clusters in the subnanometer size regime have been the topic of several computational and mass spectrometry studies that showed the particular stability of the stoichiometric species relative to nonstoichiometric species (ZnO, x ≠ y). In the current study, we present the angle-resolved anion photoelectron (PE) spectra of stoichiometric ZnO^-clusters (2 ≤ x ≤ 5), which are interpreted with supporting computational studies that include natural orbital ionization calculations on detachment transition cross sections. All spectra show evidence of D_h ring structures, which had been predicted to be the most stable structures in previous computational studies. However, a new lowest energy isomer is reported for the Zn₂O₂ anion and neutral, a bent chain, which is readily reconciled with the most intense feature in the Zn₂O₂^- PE spectrum. The computed PE angular distributions (PADs) associated with the lowest energy cluster structures identified computationally agree with the experimental results, with the exception of Zn₅O₅^-, the experimental PAD of which suggests that strong vibronic coupling may be introducing anomalies. While the lowest lying electronic state of the Zn₂O₂ chain structure is a triplet state, all neutral ring structures (including Zn₂O₂, the anion of which also populates the ion beam), favor a singlet electronic state. The computed singlet-triplet splitting of the D_h structures increases monotonically with x. Overall, we find that the properties of the ring structures evolve smoothly, rather than in the punctuated manner typically seen in the small cluster size regime.