Variations in carbonyl mode intensities in inelastic electron tunneling, and multiple reflection absorption infrared spectroscopy Part I: effects due to the lead cover films for selected mono-carboxylic acids on alumina

In the first of a series of three articles, evidence for attenuation of the carbonyl vibrational mode intensity in inelastic electron tunneling spectroscopy (IETS), and multiple reflection absorption infrared spectroscopy (MRAIRS) is presented. The work may have considerable impact on the interpretation of IET and MRAIR spectra of certain polymers adsorbed on alumina of interest in the field of adhesion. For a better comparison between the two techniques, MRAIR spectra are recorded for compounds adsorbed on conventional oxidized aluminum mirror samples and on samples prepared with thin-film lead cover films to form planar aluminum/alumina/compound/lead structures similar to the tunnel junctions used for IETS. The compounds chosen for the study are all carboxylic acids, known to adsorb on alumina by coordinate bonding of the carboxylate anion in acid–base reactions. The acids contain non-bonding carbonyl groups on the carbon backbone of the molecules whose intensities are investigated. When the compounds are constrained by the above sample geometry, similar carbonyl attenuation is observed in both IETS and MRAIRS, but to varying degrees depending on the geometry of the adsorbed molecules. The effect, which disappears in MRAIR spectra for samples without the lead cover films, can be explained by consideration of the deformation of the molecules by the lead cover film, which results in steric hindrance of the carbonyl groups and a corresponding reduction in their activity in accordance with the orientational selection rules for both IETS and MRAIRS. This and further spectroscopic evidence for deformation of the molecules by the lead cover film is discussed.