Infrared reflection–absorption spectroscopy is an optical technique used to study thin (often submonolayer) films adsorbed on reflective substrates. Experimentally, it involves measuring the change in the reflectance spectrum of the substrate that accompanies adsorption. On metal surfaces, the reflection is usually performed at grazing incidence in order to maximize sensitivity, and 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. On semiconductors, or dielectric substrates, selection rules depend strongly on the optical characteristics of the surface: vibrational modes with components vibrating in both the perpendicular and the parallel direction to the substrate can be detected, leading to absorption bands whose shape depends strongly on the experimental parameters (incidence angle, light polarization state, angle of the vibration‐induced dipole, and the nature of the substrate). Application of these surface‐induced selection rules often yields important information about adsorption geometry. Other advantages of the technique include its high sensitivity, down to the monolayer, its ability to operate at high ambient pressures or even in liquids, and the ease with which its results can be correlated with those from other vibrational spectroscopies, both surface and bulk sensitive.
