Inelastic electron tunneling spectroscopy of single ring compounds adsorbed on alumina

Studies of inelastic tunneling spectra of nitro-substituted single aromatic ring compounds have shown a correlation between tunneling mode intensity and orientation of the ring relative to the tunnel junction interface. Details of the adsorption of substituent groups on the alumina barrier can also be deduced from the tunneling spectra. In a series of halogen-substituted benzoic acids where the ring orientation is relatively constant intensity changes for specific modes can be interpreted in terms of changes in the bond character with substitution. Tunneling spectra of tetracyanoquinodimethane (TCNQ), tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), ${\mathrm{Li}}^{+}$ ${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}^{-}$, and TCNQ-${\mathrm{d}}_4$ have shown that for all derivatives the ionized form of the molecule is the species adsorbed on the alumina barrier. Analysis of the mode frequencies and intensities suggests that the Raman type of interaction makes dominant contributions to the tunneling intensity in TCNQ derivatives. Comparison of the tunneling intensities to calculated linear electron-phonon coupling constants for the TCNQ molecule shows that the magnitudes of tunneling intensity and coupling constant are correlated and can be identified with modes of the same symmetry.