We present Raman scattering spectra of CaF2samples with Ca colloids and MgO:Li with Li precipitates. The samples have been prepared by different thermal and irradiation treatment in order to achieve different colloid sizes. Exciting in the colloid surface-plasmon band we observe SERS of the whole density of states of CaF2or MgO phonons, the intensity being enhanced when the excitation approaches the colloid band maximum. In order to explain the appearance of the one phonon density of states and the resonant behaviour we propose a mechanism consisting of surface plasmon and charge transfer excitation, superposed to the electromagnetic process.