We present experimental and theoretical results for light emission from a scanning tunneling microscope probing the metallic quantum well system consisting of a Na overlayer on Cu(111). The experimental results can be summarized as follows: (i) At low bias voltages a broad, plasmon‐mediated spectrum of emitted light is seen. (ii) Once the bias voltageUis large enough that electrons can be injected from the tip into the second QWS, the spectrum becomes sharper and is centered at a photon energyhvcorresponding to the energy difference between the second and first QWS,E2−E1. (iii) Finally, and most strikingly, photons with this energy (E2−E1) are observed, albeit with lower intensity, also wheneU<hv. In this work we employ a one‐dimensional model potential to describe the electronic structure of the overlayer system, and combine this with a calculation of the electromagnetic “plasmon‐mediated” enhancement of the spontaneous emission. Furthermore the calculations show that two‐electron processes, either an Auger‐like mechanism or a hot‐electron‐hole cascade in the tip, can cause light emission in the case whenhv>eUwith quantum yield and power‐law variation of the intensity with the tunnel current in good agreement with experiment. © 2003 American Institute of Physics