Some aspects of the use of infrared photothermal beam deflection spectroscopy for studies of chemisorbed species

Abstract

Photothermal bean deflection spectroscopy (PBDS) was used to record infrared (IR) spectra of monolayers of pyridine adsorbed on particulate adsorbers which absorb IR radiation weakly, and of pyrene adsorbed on medium- and high-temperature carbons which are very strong IR absorbers. The various results show that although some IR data about surface layers on weak absorbers can be obtained, the data are generally inferior in quality than those obtainable with IR transmission/ absorption measurements. PBDS results decrease in quality as the transparency of the sample increases. Increasing the specific surface area and the degree of compaction of the sample can lead to an improvement, but a high surface area per se is neither a necessary nor sufficient condition. The scattering of IR radiation appears to be the most important of the parameters which can affect the quality of the PBD spectra, and a favourable combination of surface area, compaction and especially scattering effects can result. In general, the PBD technique has little to offer for use with non-scattering weak absorbers, but can be usefully applied with highly-scattering weak absorbers. With the other extreme type of sample, the very strong absorbers, PBDS has been used to observe monolayers of pyrene on medium- and high-temperature carbons. Although the spectra of the adsorbed species are relatively poor in comparison to those obtained with medium or strong absorbers, improvements in technique seem possible, so that PBDS will become a unique method for the study of surface effects with very strong absorbers.