Transition of the electronic response from molecular-like to jellium-like in cold, small sodium clusters

Absolute photoabsorption cross sections for Na-n(+) (2 less than or equal 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 to a temperature of similar to 105 K. The spectra for n less than or equal 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 excitations 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 greater than or equal to 4, showing that the main excitations of the valence electrons have been found.