Effect of microscopic surface roughness in surface-enhanced infrared absorption spectrometry

A surface selection rule has previously been proposed for surface-enhanced infrared absorption that is analogous to the corresponding rule for bulk metals, in that molecular vibrations whose dipole moment change is perpendicular to the surface of the metal will be observable, and those vibrations whose dipole moment change is parallel will be forbidden. In the present study, it was observed that these selection rules may not be applicable under all circumstances, due to the roughness of the metal surface. It was also postulated that there may be more than one type of adsorbate species, either due to different surface sites or due to different adsorption geometry, In some cases, the surface selection rule is obeyed, and in others it appears to be broken.