  Disentangling the contributions of radiative and non-radiative localized
plasmonic modes from the photonic density of states of metallic nanocavities
between atomically-sharp tips and flat substrates remains an experimental
challenge nowadays. Electroluminescence due to tunnelling through the
tip-substrate gap allows discerning solely the excitation of radiative modes,
but this information is inherently convolved with that of the electronic
structure of the system. In this work we present a fully experimental procedure
to eliminate the electronic-structure factors from the scanning tunnelling
microscope luminescence spectra by confronting them with spectroscopic
information extracted from elastic current measurements. Comparison against
electromagnetic calculations demonstrates that this procedure allows
characterizing the meV shifts experienced by the dipolar and quadrupolar
plasmonic modes supported by the nanocavity under atomic-scale gap size
changes. Our method, thus, gives us access to the frequency-dependent radiative
Purcell enhancement that a microscopic light emitter would undergo when placed
at the nanocavity.
