Abstract: On the basis of the nonequilibrium Green's function theory and perturbation theory, an IETS formalism is developed. In the formalism, the nonequilibrium Green's function of a molecular junction is separated into contributions from the elastic and inelastic electron tunneling, respectively. And IETS is directly related to the electron-vibrational coupling constant. The formalism provides insights into the mechanism of active vibrational modes for IETS. By applying the Gaussian 03 and DUSHIN programs to our formalism, the IETS of molecular junctions of oligo(phenylene vinylene) and oligo(phenylene ethynylene) are calculated, and good agreement between the calculation results and experimental measurements is observed.

Key words: Molecular junction, Inelastic tunneling spectroscopy, Electron-vibrational coupling, Perturbation theory, ab initio, Green’s function