Photoelectron Velocity Map Imaging Spectroscopy of the Beryllium Trimer and Tetramer.

Computational studies of small beryllium clusters (Be N ) 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 Be 2 molecule. In the present study, we report spectroscopic data for Be 3 and Be 4 obtained via anion photodetachment spectroscopy. The trimer is predicted to have D 3 h symmetric equilibrium structures for both the neutral molecule and the anion. Photodetachment spectra reveal transitions that originate from the X 2 A 2 ″ ground state and the 1 2 A 1 ' electronically excited state. The state symmetries were assigned on the basis of anisotropic photoelectron angular distributions. The neutral and anionic forms of Be 4 are predicted to be tetrahedral. Franck-Condon diagonal photodetachment was observed with a photoelectron angular distribution consistent with the expected Be 4 - X 2 A 1 → Be 4 X 1 A 1 transition. The electron affinities of Be 3 and Be 4 were determined to be 11363 ± 60 and 13052 ± 50 cm -1 , respectively.