Extracting Potentials from Spectra

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Science  19 Jun 2009:
Vol. 324, Issue 5934, pp. 1526-1527
DOI: 10.1126/science.1175751

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For most elements, we know whether they can form a diatomic molecule, especially for light atoms that have few electrons and can be treated readily by theory. But for one such light element, surprises are still in store. For most of the 20th century, experimental and theoretical studies agreed that the beryllium dimer (Be2) did not exist. The Be atom has filled electron shells and—like the inert gases such as helium—was expected to form at most a weak van der Waals dimer at very large internuclear distances. Yet, as shown experimentally by Merritt et al. on page 1548 of this issue (1), Be2 does exist and has a relatively short bond (2.45 Å), relative to the anticipated van der Waals complex with a bond length of about 5 Å. Its unusually flat potential curve limits the number of vibrational levels and provides the rare opportunity to study the highest vibrational state of a molecule just at its dissociation limit.