  A many-body Green's function approach to the microscopic theory of
plasmon-enhanced spectroscopy is presented within the context of localized
surface-plasmon resonance spectroscopy and applied to investigate the coupling
between quantum-molecular and classical-plasmonic resonances in
monolayer-coated silver nanoparticles. Electronic propagators or Green's
functions, accounting for the repeated polarization interaction between a
single molecule and its image in a nearby nanoscale metal, are explicitly
computed and used to construct the linear-response properties of the combined
molecule-metal system to an external electromagnetic perturbation. Shifting and
finite lifetime of states appear rigorously and automatically within our
approach and reveal an intricate coupling between molecule and metal not fully
described by previous theories. Self-consistent incorporation of this
quantum-molecular response into the continuum-electromagnetic scattering of the
molecule-metal target is exploited to compute the localized surface-plasmon
resonance wavelength shift with respect to the bare metal from first
principles.
