Photoelectron Velocity Map Imaging Spectroscopy of the Beryllium Trimer and Tetramer

Computational studies of small beryllium clusters (Be) predict dramatic, nonmonotonic changes in the bonding mechanisms and per-atom cohesion energies with increasing N. To date, experimental tests of these quantum chemistry models are lacking for all but the Be₂ molecule. In the present study, we report spectroscopic data for Be₃ and Be₄ obtained via anion photodetachment spectroscopy. The trimer is predicted to have D₃ symmetric equilibrium structures for both the neutral molecule and the anion. Photodetachment spectra reveal transitions that originate from the X²A₂″ ground state and the 1²A₁' electronically excited state. The state symmetries were assigned on the basis of anisotropic photoelectron angular distributions. The neutral and anionic forms of Be₄ are predicted to be tetrahedral. Franck-Condon diagonal photodetachment was observed with a photoelectron angular distribution consistent with the expected Be₄^-X²A₁ → Be₄X¹A₁ transition. The electron affinities of Be₃ and Be₄ were determined to be 11363 ± 60 and 13052 ± 50 cm^-1, respectively.