Infrared Reflection-Absorption Studies of Thin Films at Grazing Incidence

Abstract

Infrared spectra of thin films (<200 Å) on metals have been observed with Fourier Transform IR spectroscopy (FT-IR) by improvements to the grazing incidence reflection methods proposed by Greenler. Theoretical extensions to the reflectivity ratio, ( R – R0) /R0 for a number of metals have been made and indicate the optimal conditions for each experiment. Grazing incidence reflection measurements of poly(methyl methacrylate) (PMMA) films (thickness 180 Å) have been made on silver-, aluminum-, copper-, and chromium-coated substrates. Although the reflectivity ratio maximizes at an angle which deviates a few degrees from pure grazing, filling factor considerations and optimal optical fields show both experimentally and theoretically that operating some ten degrees away from grazing gives better signal-to-noise spectra for reasonable accumulation times. Furthermore, a shift in the maximum of the reflectivity ratio curve is observed from one metal to another, and a comparison between silver and chromium is used to illustrate this effect on the observed IR spectra.