Photoelectron Velocity Map Imaging Spectroscopy of the Beryllium Trimer and Tetramer

[Image: see text] 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(3h) 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.