We have investigated the inverse photoelectric effect stimulated by electron tunnelling in the scanning tunnelling microscope (STM). Light emission spectra are reported for polycrystalline silver films. In accordance with similar observations for other tunnelling devices, the high-energy threshold of the spectral distributions has been found to equal the voltage between tip and sample. Characteristic maxima of the photon emission at ∼1.9 eV and ∼2.4 eV, independent of tunnel voltage, are attributed to resonant excitation and radiative decay of localized surface plasmon polariton modes at the silver surface. This represents the first observation of plasmon excitations with the STM and provides unambiguous evidence of local inelastic processes occurring on silver surfaces. We propose that our novel results may have particular relevance to the interpretation of optical phenomena on the atomic scale. © 1989, IOP Publishing Ltd.