Spatial coherence effect on the low-frequency Raman scattering from metallic nanoclusters

The low-frequency plasmon-resonant Raman scattering by the vibrational modes of silver nanoclusters embedded in amorphous ${\mathrm{SiO}}_2$ films is studied experimentally and theoretically. By electron microscopy it is observed that the concentration of defects in nanocrystals decreases and the intensity of Raman scattering increases by thermal annealing. It is shown that the degree of spatial coherence inside clusters has a strong effect on the Raman intensity and on its frequency dependence.