Optical response of quantum-sized Ag and Au clusters - cage vs. compact structures and the remarkable insensitivity to compression

Quantum-sized silver and gold clusters show very different spectral characteristics. While silver exhibits a strong localized surface-plasmon resonance (LSPR) band down to very small sizes, the resonance is broadened beyond recognition in Au clusters below about 2 nm. In the present work, we study icosahedral hollow-shell structures, or cages, of about 1.8 nm diameter in comparison with compact clusters and show that the qualitative difference between Ag and Au remains but is reduced, as a significant increase of absorption is found for the Au cage structures. The silver shell Ag-92 exhibits a resonance that is red-shifted compared to the compact Ag-147, coinciding with the general result found in much larger shells that are amenable to the classical description by Mie theory. However, the electronic structure in particular of the d band is strongly changed. The spectrum of the empty Ag shell is remarkably similar to the spectrum of the respective Au55Ag92 core-shell structure. Inspection of the time-dependent electronic density does not explain this similarity. However, it shows that the overall classical picture of a collective charge oscillation remains valid, although clearly with modifications. We further show a remarkable insensitivity of the absorption spectra of both Ag and Au clusters to even rather extreme values of compression or dilatation.