Infrared Reflection–Absorption Spectroscopy

Abstract Infrared reflection–absorption spectroscopy (IRAS, IRRAS) is an optical technique used to study thin (often submonolayer) films adsorbed on reflective substrates such as metals. Experimentally, it involves measuring the change in the reflectance spectrum of the substrate that accompanies adsorption. In order to maximize sensitivity, the reflection is usually performed at grazing incidence. On metal surfaces, the process is subject to an overriding selection rule which states that only those vibrational modes that have a component of their dipole change perpendicular to the surface can be detected: application of this metal surface selection rule (MSSR) often yields important information about adsorption geometry. Other advantages of the technique include its high sensitivity (<10 −4 monolayers in favorable instances), its ability to operate under high ambient pressures where other primarily electron‐based surface spectroscopies cannot be utilized, and the ease with which its results can be correlated with those from other vibrational spectroscopies, both surface‐ and bulk‐sensitive. A significant current disadvantage is the difficulty in studying low‐wavenumber vibrations, but this is being overcome by the development of more intense far‐infrared sources. Various acronyms for the technique have been employed – IRAS, IRRAS and Reflection–Absorption Infrared Spectroscopy (RAIRS) have all enjoyed wide currency. The last of these is perhaps the most widely used and is the form advocated by the International Union of Pure and Applied Chemistry (IUPAC).1, 2 Accordingly, it will be adopted in this article.