Plasmon resonances in linear atomic chains: Free-electron behavior and anisotropic screening of d electrons

Electronic excitations in linear atomic chains of simple and noble metals (silver) have been studied using time-dependent density-functional theory. The formation and development of collective resonances in the absorption spectra were obtained as functions of the chain length. A longitudinal collective resonance appears in both simple- and noble-metal chains. Its dispersion has been deduced and is compared with that of a one-dimensional electron gas. The transverse excitation generally shows a bimodal structure, which can be assigned as the "end and central resonances." The d electrons of silver atoms reduce both the energies and intensities of the transverse modes but have little effect on its longitudinal resonance. This anisotropic screening is determined by the interband (d -> p) transition, which is involved only in transverse oscillations. Analysis of these results yields a general picture of plasmon resonances in one-dimensional atomic structures. Implications of such atomic-scale plasmons to surface plasmons in larger dimensions are also discussed.