Vibrational-mode shifts in inelastic electron tunneling spectroscopy: Effects due to superconductivity and surface interactions

Measurements of the effect of the top metal electrode on the vibrational-mode energies of benzoic acid on aluminum oxide with Pb, Sn, and Ag top metal electrodes are described. Peak shifts of approximately 15 meV for the O-H stretch modes of hydroxyl ions, 0.5 meV for the C-H bending modes, and less than 0.2 meV for the C-C ring stretch and bend modes of benzoate molecules were found. These measurements are discussed in terms of a simple image model. For junctions with superconducting electrodes a modulation-dependent correction must be made to the measured peak positions. A theory is developed for the line shapes and peak positions of both the second and third harmonic signals and compared with experiment.