STM light emission (STML) has been shown to be a promising technique for nanometer scale analyses of electronic states of molecules on metals as the light emission due to the electronic transition between molecular levels is comparably intense with the plasmon induced light emission for noble metals. The energy forbidden light emission in the spectra observed for porphyrin molecules on Au has been discussed on the basis of strong coupling between molecules and plasmon field in STM nano-cavity. The substrate material dependence of plasmon enhancement is well reproduced by an electromagnetic calculation using the effective dielectric constant for the molecular film/substrate material.