  Surface-Enhanced Raman Scattering (SERS) allows for detection and
identification of molecular vibrational fingerprints in minute sample
quantities. The SERS process can be also exploited for optical manipulation of
molecular vibrations. We present a quantum description of Surface-Enhanced
Resonant Raman scattering (SERRS), in analogy to hybrid cavity optomechanics,
and compare the resonant situation with the off-resonant SERS. Our model
predicts the existence of a regime of coherent interaction between electronic
and vibrational degrees of freedom of a molecule, mediated by a plasmonic
nanocavity. This coherent mechanism can be achieved by parametrically tuning
the frequency and intensity of the incident pumping laser and is related to the
optomechanical pumping of molecular vibrations. We find that vibrational
pumping is able to selectively activate a particular vibrational mode, thus
providing a mechanism to control its population and drive plasmon-assisted
chemistry.
