Molecular Vibration Spectra by Inelastic Electron Tunneling

An experimental and theoretical study has been carried out on the process of inelastic tunneling between metal films. The experimental results show that this process gives rise to the observation of vibrational spectra for molecules contained in the junction region. The phenomena have been studied for a number of molecular species and at temperatures ranging from 1 to 300\ifmmode^\circ\else\textdegree\fi{}K. The principal effect of temperature is to broaden the observed spectral lines which can be accounted for quantitatively by considering the thermal smearing of the electron distribution in metals. Two detailed interaction mechanisms are discussed which can give rise to infrared dipole or Raman selection rules. None of the experiments carried out thus far can distinguish between these two processes, because of the low symmetry of the molecules used. In addition to the vibrational spectra, it appears that it should be possible to observe electronic transitions as well.