C. Ellert et al., TRANSITION OF THE ELECTRONIC RESPONSE FROM MOLECULAR-LIKE TO JELLIUM-LIKE IN COLD, SMALL SODIUM CLUSTERS, Zeitschrift fur Physik. D, Atoms, molecules and clusters, 39(4), 1997, pp. 317-323
Absolute photoabsorption cross sections for Na-n(+) (2 less than or eq
ual to n less than or equal to 21) were measured in the visible energy
range. The cluster ions were produced in a gas aggregation source and
thus have a canonical distribution of internal energy corresponding t
o a temperature of similar to 105 K. The spectra for n less than or eq
ual to 9 and 11 exhibit between two and six absorption lines, and are
in qualitative agreement with ab inito quantum chemical calculations.
For n=15 and 21, the position of the resonances can be explained as ex
citations of a nearly free electron gas in a spheroidal container. An
evolution is thus observed from molecular-like transitions to a giant
collective resonance of the electron cloud. The integrated oscillator
strength is 0.95 per 3s-electron in the energy range covered for n gre
ater than or equal to 4, showing that the main excitations of the vale
nce electrons have been found.