Reflection Method for Obtaining the Infrared Spectrum of a Thin Layer on a Metal Surface

TLDR

Infrared absorption of a thin, moderately absorbing layer on a metal surface varies strongly with incidence angle and polarization. The study defines spectroscopic system requirements for measuring such layers under realistic multiple‑reflection, low f/number beam conditions. The method uses closely spaced mirrors to generate multiple reflections, with radiation emerging from a narrow aperture that acts as an effective source for the infrared monochromator. The constructed system successfully produced the infrared spectrum of a cellulose acetate layer on a silver mirror.

Abstract

Previous calculations have shown that a thin, moderately absorbing layer on a metal surface has an absorption factor in the infrared which varies greatly with the angle of incidence and the state of polarization of the incident radiation. The requirements on a spectroscopic system, which uses the insights of these calculations, are discussed under the realistic conditions of multiple reflections with a low f/number beam. A method is described for obtaining many reflections between closely spaced mirrors. Radiation emerges from a narrow aperture between the mirrors, the aperture serving as an effective source for the infrared monochromator. A system has been constructed and its use is illustrated by the spectrum of a cellulose acetate layer on a silver mirror.