  A molecular wire containing an emitting molecular center is controllably
suspended between the plasmonic electrodes of a cryogenic scanning tunneling
microscope. Passing current through this circuit generates an ultra narrow-line
emission at an energy of ? 1.5 eV which is assigned to the fluorescence of the
molecular center. Control over the linewidth is obtained by progressively
detaching the emitting unit from the surface. The recorded spectra also reveal
several vibronic peaks of low intensities that can be viewed as a fingerprint
of the emitter. Surface-plasmon localized at the tip-sample interface are shown
to play a major role on both excitation and emission of the molecular excitons.
