High-Resolution Photoelectron Imaging of Cryogenically Cooled BiB₂^- and BiB₃^- Bismuth-Boron Clusters

We report a high-resolution photoelectron imaging study of cryogenically cooled BiB₂^- and BiB₃^- clusters. Vibrational features are completely resolved for the ground-state detachment transitions, providing critical information about the structures of the anionic clusters and their corresponding neutrals. The electron affinities of BiB₂ and BiB₃ are accurately measured to be 2.174(1) and 2.121(1) eV, respectively. The B-B and Bi-B stretching frequencies are measured to be 1262 and 476 cm^-1, respectively, in the ground state of BiB₂. Three vibrational frequencies are measured for the ground state of BiB₃: 1194 cm^-1 (B-B stretching), 782 cm^-1 (B-B stretching), and 339 cm^-1 (Bi-B stretching). Both BiB₂^- and BiB₃^- and their neutral ground states are found to have planar C₂ structures in which the Bi atom bridges two B atoms. BiB₂^- is found to have a triplet spin state (³B₂), consistent with its complicated photoelectron spectra, whereas BiB₃^- is a doublet (²B₁) and neutral BiB₃ is closed shell (¹A₁). Both BiB₂ and BiB₃ consist of peripheral localized Bi-B and B-B σ bonds and delocalized π and σ bonds.