  In this work we identify coherent electron-vibron interactions between
near-resonant and non-resonant electronic levels that contribute beyond
standard optomechanical models for off-resonant or resonance SERS. By
developing an open-system quantum model using first molecular interaction
principles, we show how the Raman interference of both resonant and
non-resonant contributions can provide several orders of magnitude
modifications of the SERS peaks with respect to former optomechanical models
and over the fluorescence backgrounds. This cooperative optomechanical
mechanism allows for generating an enhancement of nonclassical photon pair
correlations between Stokes and anti-Stokes photons, which can be detected by
photon-counting measurements. Our results demonstrate Raman enhancements and
suppressions of coherent nature that significantly impact the standard
estimations of the optomechanical contribution from SERS spectra and their
quantum mechanical observable effects.
