  Scanning tunneling microscope induced light emission from an atomic or
molecular junction has been probed from the tunneling to contact regime in
recent experiments. There, the intensity of the light emission shows strong
correlation with the current/charge fluctuations at optical frequencies. We
show that this is consistent with the established theory in the tunneling
regime, by writing the finite-frequency shot noise as a sum of inelastic
transitions between different electronic states. Based on this, we develop a
practical scheme to perform calculations on realistic structures using Green's
functions. The photon emission yields obtained re-produce the essential feature
of the experiments.
