  Interaction between plasmonic nanostructures and molecules is modeled based
on the concept of quantized optical cavity for surface enhanced Raman
scattering process. We have found that the background emission from plasmonic
nanostructures is not constant as speculated ordinarily, it is enhanced
accompanying with the molecules Raman scattering. The plasmonic nanostructures
not only scatter elastically the energy coupling from the molecules excited
states, but also radiate it inelastically as surface plasmon emission partly
resulting an enhanced background. According to single nanoparticle experiments,
the model reveals that the background fluctuations is mainly due to the induced
field of the molecules, which increases the local field felt by the
nanostructures that was often overlooked in the past. These findings suggest
considering the plasmonic nanostructures and molecules as a hybrid entity to
analyze and optimize the surface enhanced spectroscopy.
