Two-Electron Photon Emission From Metallic Quantum Wells
  Unusual emission of visible light is observed in scanning tunneling microscopy of the quantum well system Na on Cu(111). Photons are emitted at energies exceeding the energy of the tunneling electrons. Model calculations of two-electron processes which lead to quantum well transitions reproduce the experimental fluorescence spectra, the quantum yield, and the power-law variation of the intensity with the excitation current. 
Germar Hoffmann (1), Richard Berndt (1), and Peter Johansson (2) ((1)   University of Kiel, Germany; (2) University of Orebr...