Structural and Electronic Properties of Anionic (ThO₂)_n^- (n = 2-4) Clusters

Thorium dioxide nanomaterials have attracted broad interest due to their catalytic properties and their possible use as fuel in next generation nuclear reactors. Investigation of their chemical and physical properties benefits from exploration of the electronic structure of small cluster units. A joint computational and experimental study is reported herein of the geometric and electronic structure of the neutral and anionic thorium dioxide clusters (ThO₂)_n^0/-, n = 2, 3, 4. Differences were found in the identity of the global minimum structure and the distribution of structural isomers between the neutral clusters and their anionic counterparts at each size that can be traced to the nature of the highest occupied molecular orbital (HOMO). The computed vertical detachment energy (VDE) value of each cluster was in excellent agreement with the first peak of the experimental anion photoelectron spectroscopy (aPES) spectra. This spectral feature was identified as corresponding to electron ionizations from the HOMO of the global minimum structure in the (ThO₂)_n^- (n = 2, 3, 4) clusters. To explore the origin of spectral features in the measured spectra of (ThO₂)_n^- (n = 2, 3, 4) beyond the vertical detachment energy (VDE) peaks, a quantitative evaluation of the existing isomers was performed based on their Boltzmann distribution ratios. These fine spectral details were partially attributed to the contribution of structural isomers beyond the global minimum.