Calculation of adsorption-induced differential external reflectance infrared spectra of particulate metals deposited on a substrate

Abstract

The existence of inverted IR bands in both external and internal reflectance infrared (IR) spectra of CO chemisorbed on particulate metals deposited on a moderately IR-reflecting substrate is well documented. This apparently striking phenomenon, in which the intensity of the reflected IR beam is higher in the presence of CO than in its absence, is in fact a simple consequence of Fresnel’s reflectance laws, as already shown in a former work [J. Electroanal. Chem. 529 (2002) 145] in which the effect of the CO adsorbate was assumed to be a small, arbitrarily fixed change of the complex refractive index of a thin layer of metal particles embedded in a dielectric matrix, this thin layer being modelled in the framework of Bruggeman’s effective medium theory. In the present work, using a simple yet realistic model for the adsorption of CO on a metal particle, and an advanced effective medium model [Phys. Rev. B 49 (1994) 7137] for the thin layer of metal particles, we have been able to calculate quantitatively the small changes of the refractive index of the thin layer due to the adsorption of CO, and therefore to calculate, also quantitatively, the shape and intensity of the bands that should be observed. Again, inverted bands are predicted to appear under certain circumstances, which shows that no new phenomenon need be invoked for explaining the appearance of these inverted bands, but only the well-known Fresnel’s reflectance laws. Obviously, since the intensity of the bands is quantitatively calculated, the magnitude of the enhancement (when present) of the intensity of the adsorbate bands, i.e., the magnitude of the surface enhancement of infrared absorption (SEIRA) effect, is obtained. Importantly, both band inversion and SEIRA are found to depend on the fill factor (volume fraction of metal particles in the thin layer), and not on the size of the metal particles.