High resolution photoelectron imaging of boron-bismuth binary clusters: Bi₂B_n ^- (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, Bi₂B_n ^- (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_∞h, ²Π_g) with a B₂ unit and two terminal Bi atoms, while Bi₂B₃ ^- is found to be planar (C_2v, ¹A₁), consisting of a B₃ 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 B₄ unit with two bridging Bi atoms in a trans (C_2h, ²A_u) and cis (C_2v, ²B₁) 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.