Photodepletion probes of Na5, Na6, and Na7. Molecular dimensionality transition (2D→3D)?

Photodepletion spectroscopy was used to obtain electronic absorption data for Na5, Na6, and Na7 in a wavelength range from 420–770 nm. Measurements were rationalized using several models, ranging from classical electrostatic to ab initio large-scale configuration-interaction calculations. As in previous studies of Na4, Na8, and Na20, classical Mie–Drude theory applied to appropriately dimensioned metal droplets describes the gross but not a number of fine-structure features of the spectra observed. In contrast, ab initio large-scale configuration-interaction predictions of vertical electronic excitations from specific ground-state geometries can more fully account for the measurements. Theory–experiment comparison suggests that Na6 is predominantly formed under our molecular-beam conditions in a planar or near-planar structure. In contrast, the spectrum obtained for Na7 is consistent with a three-dimensional topology.