Optical absorption spectra of intermediate-size silver clusters from first principles

Optical absorption spectra of the three lowest-energy isomers of ${\text{Ag}}_n$ $\left(n=10,12–20\right)$ are investigated from first principles within the time-dependent local-density approximation (TDLDA). The computed spectra are found to be generally in good agreement with the available experimental data. The analyses of the spectra indicate that the $d$ electrons of ${\text{Ag}}_n$ clusters in this size range have a significant $\left(70\%–80\%\right)$ contribution to low-energy optical excitations. We show that most of the peak positions and the relative intensities in the TDLDA spectra of these subnanometer sized clusters can be explained remarkably well within the classical Mie-Gans theory, using the dielectric function of bulk Ag and taking into account the shapes of the isomers.