High resolution photoelectron imaging of boron-bismuth binary clusters: Bi2Bn- (n=2-4)

Bismuth boride is a heavy member of the III-V semiconductors. Although there have been some theoretical interests in this material, it has not been synthesized experimentally. Here, we report a high-resolution photoelectron imaging study on a series of boron-bismuth binary clusters, Bi2Bn- (n = 2-4), produced by laser vaporization of a B/Bi mixed target. Vibrationally resolved photoelectron spectra are obtained for all three clusters, and the measured vibrational and electronic information is used to compare with theoretical calculations to understand their structures and bonding. Bi2B2- is found to be linear (D-infinity h, (2)Pi(g)) with a B-2 unit and two terminal Bi atoms, while Bi2B3- is found to be planar (C-2v, (1)A(1)), consisting of a B-3 triangle with two bridging Bi atoms. Interestingly, the spectra of Bi2B4- reveal two co-existing isomers; both are found to be planar and contain a rhombus B-4 unit with two bridging Bi atoms in a trans (C-2h, Au-2) and cis (C-2v, B-2(1)) fashion separated only by 0.03 eV in energy. The interactions between the two Bi atoms and the B-n motifs are understood using chemical bonding analyses. This study shows that the Bi-B bonding is weak enough so that the B-n units maintain their structural integrity with the Bi atoms bonded to the cluster periphery only.