Theoretical Interpretation of IETS Data
We first discuss the initial theoretical work on IETS by SCALAPINO and MARCUS, which included a dipole potential of the molecule and its image in the tunneling barrier potential and calculated the excess tunneling current due to this potential within the WKB approximation for infrared active modes. The theory obtained the proper intensity for the O-H stretch mode on alumina and predicted that the orientation of the molecules on the surface could be inferred from the tunneling spectrum, since only dipole moments oscillating perpendicular to the surface coupled to the tunneling electron. JAKLEVIC and LAMBE extended this theory to Raman modes by including the bond polarizabilities, and predicted that Raman modes should be observable in IETS with intensities about 2–5 times smaller than the infrared modes. KLEIN et al. used a transfer Hamiltonian formalism to show that LO as well as TO phonons could be observed in MgO using IETS. KIRTLEY, SCALAPINO, and HANSMA applied the transfer Hamiltonian formalism to molecular vibrations in IETS. They replaced the dipole approximation with a set of partial charges located on the atoms in the molecule, and allowed for off-axis electron scattering. In this theory the local nature of the electron-molecular interaction weakened the orientation and symmetry selection rules. It predicted that the Raman modes should be observable even neglecting bond polarizabilities, and that optically forbidden modes, although weak, may be observable. We discuss the formal difficulties involved with the theory at present, compare our predictions with experiment, and try to point the way toward further development of the theory of intensities in IETS.
KeywordsVibrational Mode Tunneling Electron Raman Mode Dipole Potential Schroedinger Equation
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