Interband effect in the optical response of strontium clusters

Photoabsorption spectroscopy has been performed on singly charged strontium clusters Srₙ⁺ for the sizes $n=61,$ 127, and 200, in the 1.8 to 5.2 eV energy range. Superimposed on the giant resonance, related to the Mie plasma resonance of a metallic sphere, the spectra reveal an extended wing in the high-energy range attributed to interband transitions. Evidence of striking correspondence with the absorption spectrum of a macroscopic sphere obtained from the measured real and imaginary parts of the bulk strontium dielectric function is shown.