First Principles Absorption Spectra of Intermediate Size Ag$_n$ ($n=10-20$) Clusters

First principles optical absorption spectra, obtained within time-dependent density functional theory (TDDFT), for the ground state and low-energy isomers of Ag$_n$ ($n=10-20$) are presented. Overall, our theoretical results exhibit quite good agreement with spectra obtained for Ag clusters trapped in rare- gas matrices. We show that the classical predictions from Mie- Gans theory using the bulk dielectric function of Ag are in rather good agreement with experimental results and TDDFT spectra for isomers of various shapes. We analyze the orbital character of the optical excitations, and unexpectedly find that the $d$ electrons of Ag$_n$ clusters in this size range have a significant contribution to low-energy optical excitations, unlike the case for smaller Ag$_n$ ($n < 9$) clusters.