Time-dependent density functional theory studies of plasmons in parallel double sodium atomic chains

The photoabsorption spectra of linear double Na atomic chains with different lengths and inter-chain spacings are investigated using the time-dependent density functional theory (TDDFT) in the frequency domain. The formation and development of the collective resonances in the spectra are studied in terms of the spacing and length. With the decreasing spacing, the enhanced coupling between the chains lead to the emergence of extra peaks. For the well coupled double chains, the excitations show a single longitudinal mode but twin bimodal structure for the transverse modes. Our calculation reveals the formation and evolution of plasmon excitations in atomic nanostructures.