Comparison of experimental and theoretical inelastic electron tunneling spectra for formic acid

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Abstract
The theoretical and experimental inelastic electron tunneling intensities were compared for several modes of formic acid adsorbed on aluminum oxide. The general features of IETS are given and experimental techniques described. Several of the proposed theoretical models and calculated results are reviewed. Calibrated spectra for formic acid and its deuterated forms were measured and the vibrational modes identified. The spectra indicated that formic acid adsorbs on alumina as a formate ion. The experimental peak intensities from the undeuterated formic acid spectrum were compared with theoretical intensities calculated using the partial-charge model. The intensities were calculated for two orientations of the formate ion, with the C-O bond perpendicular to, and parallel to, the oxide surface. Comparison of the theoretical and experimental intensities for the C-O and C = O modes indicated that the formate ion is oriented perpendicular to the oxide surface. Experimental intensities for modes involving hydrogen were not in good agreement with those obtained from the partial-charge calculations. The differences were not unexpected due to the charge distortion along the C-H bond. The results for modes involving hydrogen indicated that a more sophisticated model than the partial-charge model will be required for accurate intensity calculations for C-H modes.