  We extend the Purcell's original idea [Phys. Rev. \textbf{69}, 682 (1946)] on
modification of photon spontaneous \textit{emission} rate to modification of
photon spontaneous \textit{scattering} rate. We find the interplay of local
incident field enhancement and local density of photon states enhancement in
close proximity to a silver nanoparticle may result in up to $10^{14}$-fold
rise of Raman scattering cross-section. Thus single molecule Raman detection is
found to be explained by consistent quantum electrodynamic description without
any chemical mechanism involved. A model of the so-called "hot points" in
surface enhanced spectroscopy has been elaborated as local areas with high
Q-factor at incident and scattered (emitted) light frequencies. For
verification of the model we consider further experiments including transient
Raman experiments to clarify incident field enhancement and scanning near-field
optical mapping of local density of photon states.
