Electrons on the surface of metal nanoparticles can be represented by an electron liquid. The waves on the surface of the electron liquid are plasmons. The crystal, as a whole, is electrically neutral. The displacement of a negatively charged electron liquid (plasma) relative to a positively charged crystal lattice leads to oscillations of electric polarization. These vibrations can be induced by light of the appropriate frequency. Substances where there are plasmons and which interact with a molecule adsorbed on the surface give the SERS spectrum.

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