  The spontaneous emission of radiation of metallic electrons embedded in a
high-intensity enhanced surface plasmon field is considered analytically. The
electrons are described by exact dressed quantum states which contain the
interaction with the plasmon field non-perturbatively. Considerable deviations
from the pertubative behaviour have been found in the intensity dependence of
the emitted fundamental and the second harmonic signals, even at moderate
incoming laser intensities. The theoretical predictions deduced from the
formalism are in good qualitative agreement with the experimental results.
