Optical generation of collective plasmon modes in small gold chains induced by doping transition-metal impurities

Our examination of the optical properties of small gold chains containing up to 24 atoms doped with a transition metal (TM) atom (Ni, Rh, Fe), using the time-dependent density functional theory, show the splitting of the collective plasmon peak. We associate the additional peak with a local plasmonic mode which corresponds to charge oscillations around the potential created by the d orbitals of the impurity atoms. The effect is almost independent of the position of the TM atom in the chain, as long as it is not at the chain edge. This behavior is opposite to that of larger noble-metal-TM clusters (radius >1 nm), in which doping with TM atoms does not lead to generation of new modes, and often produces a suppression of the main plasmon peak.