  The spectral relationships between surface enhanced resonant Raman scattering
(SERRS) and plasmon resonance observed in far field scattering cross are
investigated using single silver nanoparticle dimers with focusing on the
lowest energy (superradiant) plasmon resonance. We find that these
relationships can be classified into two types. The first is SERRS spectral
envelopes with spectral shapes similar to those of plasmon resonance spectra.
The second is SERRS envelopes exhibiting higher energy shifts from the plasmon
resonance spectra. These results are examined as an effect of degree of
morphological asymmetry in dimers based on an electromagnetic (EM) mechanism.
The analysis of the first and second types reveals that dipole-dipole and
dipole-quadrupole coupled plasmon resonance (subradiant Fano resonance)
respectively determine the EM enhancement. This mechanism is commonly important
for the development of plasmonic nanostructures for various surface enhanced
spectroscopies.
