Anomalous inverse absorption features in the far-infrared RAIRS spectra of SnCl4 on thin-film SnO2 surfaces

Abstract

As part of a study of the chemistry of tin oxide chemical vapor deposition precursors at oxide surfaces, we have utilized the so-called buried metal layer approach to obtain far-IR reflection absorption infrared spectroscopy (RAIRS) spectra for SnCl4 adsorbed at a tin oxide surface supported on a polycrystalline tungsten foil. Two types of surface preparation—namely, sputtering with Ar or O2—have been used to clean the tin oxide surfaces prior to experiment. On O2-sputtered surfaces at 300 K, the spectra are dominated by an inverse-absorption feature in the form of a positive-going band in the spectrum, obtained via the ratio of the sample spectrum to that of the clean surface background. At low temperature, the spectra display both the positive-going inverse-absorption feature and the normal negative-going absorption features common to many RAIRS studies from metallic surfaces. On an Ar-sputtered surface, the inverse absorption band is not observed, but is replaced, at least at 300 K, by an absorption band at the same wave number. Simple models are employed to demonstrate how these features may arise from an analysis of the Fresnel equations for the system, and the data are discussed in terms of the possible models for inverse-absorption features that appear in the literature. It is concluded that the features observed are induced by the presence of the adsorbate, but are characteristic of the underlying oxide substrate.