  Understanding tunneling from an atomically sharp tip to a metallic surface
requires to account for interactions on a nanoscopic scale. Inelastic tunneling
of electrons generates emission of photons, whose energies intuitively should
be limited by the applied bias voltage. However, experiments by Schull et al.
[Phys. Rev. Lett. 102, 057401 (2009)] indicate that more complex processes
involving the interaction of electrons with plasmon polaritons lead to photon
emission characterized by over-bias energies. We propose a model of this
observation in analogy to dynamical Coulomb blockade, originally developed for
treating the electronic environment in mesoscopic circuits. We explain the
experimental finding quantitatively by the correlated tunneling of two
electrons interacting with an LRC circuit modeling the local plasmon-polariton
mode. To explain the over-bias emission, the non-Gaussian statistics of the
tunneling dynamics of the electrons is essential.
