First Principles Absorption Spectra of Cu$_n$ ($n=1-10$) Clusters

First principles optical absorption spectra, obtained within time-dependent density functional theory, for the ground state and low-energy isomers of Cu$_n$ ($n = 1- 10$) are presented. Overall our theoretical results exhibit good agreement with available experimental data. We analyze the orbital character of the optical excitations as a function of size and energy. Compared to noble metal clusters of Ag and Au in the same size range,\footnote {J. C. Idrobo {\em et al.} Phys. Rev. B {\bf 76}, 205422 (2007); K. Baishya {\em et al.} {\em ibid.} {\bf 78}, 075439 (2008).} we find that Cu clusters have much higher d-electron contribution to low-energy optical excitations.