Elusive bond elucidated

Experimental measurements, in agreement with theoretical calculations, have determined the elusive electronic ground state of the beryllium dimer

Credit: © 2009 AAAS

Theoretical calculations can predict the structure and properties of increasingly large molecules — yet understanding the electronic ground state of the beryllium dimer, with its eight electrons, has remained challenging. Numerous models have suggested different bond lengths and dissociation energies, but the few samples obtained experimentally could not provide sufficient information to elucidate the dimer's structure. Now, Michael Heaven at Emory University, Atlanta, and co-workers have measured¹ all the bound vibrational energies of the electronic ground state by stimulated emission spectroscopy.

The researchers used a pulsed laser to populate a single rotational–vibrational level of a particular electronically excited state (A¹Π_u), from which transitions to the vibrationally excited levels of the ground state are favoured. The relaxation of the system is then induced by a second laser, and the resulting fluorescence is recorded.

Subsequent analysis enabled the determination of the dissociation energy and the unusually shaped potential energy curve of the dimer for Be–Be distances shorter than 8.5 Å. Theoretical analyses agreed well with experiment and showed a fine balance between the chemical and physical interactions of the beryllium atoms. The study revealed a weak Be–Be bond but with a length close to that of a conventional covalent bond.