Absorption spectra of lithium clusters containing four to eight atoms have been measured using depletion spectroscopy. Few intense transitions are observed, always located in two predominant spectral regions, ~480 and 680 nm. The spectra are interpreted using ab initio configuration interaction (CI) calculations, leading to a complete characterization of the excited states and a straightforward determination of the ground state geometrical structure. Intense transitions are explained by interference effects in the transition amplitude and symmetry considerations. Comparisons with semiclassical models, in which an effective mass correction is introduced, are also presented.

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