The study of carbon clusters, especially the discovery of C60, significantly advanced the field of nanoscience. In contrast, boron (2s22p1) is electron-deficient and exhibits distinct chemistry. Over the past two decades, our group has systematically investigated pure and metal-doped boron clusters using anion photoelectron spectroscopy with a magnetic bottle detector. Most recently, we discovered that B80 adopts a buckyball structure analogous to C60. However, due to the limited resolution of the magnetic bottle spectrometer and insufficient cooling in the supersonic expansion, vibrational features were typically unresolved in our spectra. To overcome this, our group developed a high-resolution photoelectron imaging (PEI) apparatus. More recently, we significantly enhanced the setup by coupling a cryogenic ion trap with a laser vaporization source to produce cold molecular anions.

In this talk, I will discuss the construction and operation of the cryogenic ion trap. Its capabilities and potential are demonstrated through high-resolution PEI of a series of boron and doped boron clusters. We began with the diatomic molecule bismuth boride (BiB⁻), which exhibits a surprisingly complex electronic structure due to its open-shell character and strong spin-orbit coupling. We also obtained vibrationally resolved photoelectron spectra of BiB₂⁻ and BiB₃⁻, illustrating how Bi doping strengthens B–B bonds by filling the p bonding orbitals in B₂. In addition, we revisited the pure boron cluster B₈⁻ and its mono-carbon-doped analog CB₇⁻, where high-resolution PEI revealed interesting vibronic coupling and Jahn–Teller splitting effects.