High-Resolution Photoelectron Imaging of Cryogenically Cooled BiB 2 - and BiB 3 - Bismuth-Boron Clusters.

We report a high-resolution photoelectron imaging study of cryogenically cooled BiB 2 - and BiB 3 - 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 BiB 2 and BiB 3 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 BiB 2 . Three vibrational frequencies are measured for the ground state of BiB 3 : 1194 cm -1 (B-B stretching), 782 cm -1 (B-B stretching), and 339 cm -1 (Bi-B stretching). Both BiB 2 - and BiB 3 - and their neutral ground states are found to have planar C 2 v structures in which the Bi atom bridges two B atoms. BiB 2 - is found to have a triplet spin state ( 3 B 2 ), consistent with its complicated photoelectron spectra, whereas BiB 3 - is a doublet ( 2 B 1 ) and neutral BiB 3 is closed shell ( 1 A 1 ). Both BiB 2 and BiB 3 consist of peripheral localized Bi-B and B-B σ bonds and delocalized π and σ bonds.