Abstract
Measurements of light emission from a scanning tunneling microscope probing a Na overlayer on the (111) surface of Cu are reported along with the results of a model calculation that essentially agree with the experimental ones. The observed light-emission spectra show two characteristic features depending on the bias voltage. When the bias voltage is smaller than the energy of the second quantum well state formed outside the Na overlayer the light emission is due to a plasmon-mediated process, while for larger biases light emission is mainly caused by quantum well transitions between the two levels.