Electron-induced photon emission above the quantum cutoff due to time-energy uncertainty

The light emission from a tunneling junction induced by tunneling electrons has been studied around the cutoff at hν=eV_t. The emitted photons are found to exceed the excitation energy provided by the energy of the tunneling electrons. The experiments have been performed by a low- temperature scanning tunneling microscope at 80 K for an Ag(111) surface and an Ag-covered PtIr tip. A detailed analysis of the emission spectra reveals that the findings cannot be explained by the thermal broadening of the electron Fermi distribution alone. However, a correct description is found if a finite lifetime of the excited states in the range of 30-80 fs is included.