Spectral Signatures of Neutral Boron Oxide Clusters Containing Key Structural Units of the Vitreous State

Spectroscopic characterization of neutral boron oxide clusters is insightful for understanding the structures and properties of the bulk but has proven to be extremely challenging due to the difficulty in size selection. Here, we report a size-specific infrared spectroscopy study of a series of neutral boron oxide clusters using near-threshold photoionization with a tunable vacuum ultraviolet free electron laser. Quantum chemical calculations were carried out to understand the structures and bonding of the clusters and to help assign the experimental spectral features. The BO₃, B₂O₄, and B₃O₆ clusters focused in this study are found to have planar structures with BO, BO₃, and B₂O₅ groups, which are key structural units in the two-dimensional network of the vitreous state. Chemical bonding analyses revealed structural stability arising from the synergy among the terminal B≡O groups and B-O σ bonds. This work provides spectral signatures for the key structural units of the bulk and paves the way for systematic studies on the stepwise formation and growth mechanisms of boron oxide materials.