Abstract
We present experimental and theoretical results for light emission from ascanning 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‐mediatedspectrum of emitted light is seen. (ii) Once the bias voltage U is large enough that electrons can be injected from the tip into the second QWS, the spectrumbecomes sharper and is centered at a photon energy hv corresponding 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 when eU < 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 when hv > eU with quantum yield and power‐law variation of the intensity with the tunnel current in good agreement with experiment.