  The detection of fluorescence with submolecular resolution enables the
exploration of spatially varying photon yields and vibronic properties at the
single-molecule level. By placing individual polycyclic aromatic hydrocarbon
molecules into the plasmon cavity formed by the tip of a scanning tunneling
microscope and a NaCl-covered Ag(111) surface, molecular light emission spectra
are obtained that unravel vibrational progression. In addition, light spectra
unveil a signature of the molecule even when the tunneling current is injected
well separated from the molecular emitter. This signature exhibits a
distance-dependent Fano profile that reflects the subtle interplay between
inelastic tunneling electrons, the molecular exciton and localized plasmons in
at-distance as well as on-molecule fluorescence. The presented findings open
the path to luminescence of a different class of molecules than investigated
before and contribute to the understanding of single-molecule luminescence at
surfaces in a unified picture.
