EXTERNAL REFLECTION INFRARED-SPECTROSCOPY AT METALLIC, SEMICONDUCTOR, AND NONMETALLIC SUBSTRATES .1. MONOLAYER FILMS

The monolayers structure, orientation, and chemical interaction at metallic and nonmetallic substrates have been studied by infrared reflection spectroscopy. The reflection spectra of a thin film deposited on different substrates are generally modified by the optical effects as compared to the corresponding transmission spectra of the same film without a substrate support. Distinguishing the changes observed in reflection spectra caused by optical effects is crucial for the interpretation of the spectroscopic data and must be done before relating any differences in band shape, position, and intensity to structural and/or chemical bonding changes in the thin film. Therefore, spectral simulation has been used extensively to determine the optical effects phenomena. It is shown that physical insight into the mechanism of the complex optical phenomena of the interaction of the incident and reflected radiation is gained from the optical consideration of the electric field, [E2], components within the characterized layer over a wide range of optical constants of individual phases in the multilayer system. It was found that the reflection spectra are significantly changed due to optical effects recorded in the conditions in which interaction of the normal to the interface [E(Z2)] component within a thin film dominates, while for the conditions in which the parallel field components, [E(X2)] and [E(Y2)], dominate, the reflection spectra are similar to the corresponding transmission spectra. In the latter case the negative absorbance bands (reverse spectra) are expected. Experimental and simulated spectra of self-assembled cuprous ethyl xanthate films on copper, cuprous sulfide, and water are discussed for various experimental conditions. Understanding the basis of optical effects allows a more detailed interpretation of the experimental spectroscopic data. More precisely, assignment of the absorbance bands, the concepts of reorganization of molecules in a thin film, or the formation of an adsorbed film having an oriented polymeric-like structure could only be postulated on the basis of a close interpretation of the reflection spectra after carefully analyzing the optical effects. Furthermore, theoretical examination allows one to predict the optimum experimental conditions under which maximum sensitivity can be obtained, which is particularly important for the recording spectra at submonolayer coverages.