Variations in carbonyl mode intensities in inelastic electron tunneling and multiple reflection absorption infrared spectroscopy. Part III: effects due to surface coverage of adsorbates

In this paper, the last in the series of three articles, inelastic electron tunneling (IET) spectra are obtained for a series of aliphatic dicarboxylic acids of increasing chain length with one of the methylene groups substituted by a carbonyl group which is not involved in bonding with the alumina surface. The effect of surface coverage on the relative intensity of the carbonyl mode is discussed. For lower surface coverages, the number of possible configurations for the adsorbed molecules increases, and in certain cases, the carbonyl group is located closer to the lead cover film thereby attenuating its intensity. The effect of changing location of the carbonyl group within the molecule (and hence, the tunnel barrier) and increasing chain length on the relative intensity of the carbonyl mode is also discussed.