  Nonlinear electron emission processes induced by surface plasmon oscillations
have been studied both experimentally and theoretically. The measured
above-threshold electron spectra extend up to energies whose appearance cannot
be explained solely by standard non-perturbative methods, which predict photon
energy separated discrete energy line spectra with the known fast fall -
plateau - cutoff envelope shape, even when taking the large field enhancement
into account. The theoretical analysis of our data, based on the concept of
plasmon-induced near-field effects, gives reasonably good explanation and
qualitative agreement in the whole intensity range.
