V. M. Rudoy’s research while affiliated with Russian Academy of Sciences and other places

By expanding Fresnel's formulae in McLaurin's series of the extinction coefficient (k2), expressions for the absorbances in the ATR spectra are obtained. It is shown that in the case of weak absorption the absorbance values are directly proportional to the values of the imaginary part of the dielectric constant (ε''2). Thus, the ATR spectra obtained under conditions of weak absorption are, in effect, dependences of ε''2 on the frequency (nu). It is also shown that the use of the effective thickness concept, introduced by Harrick, leads to an incorrect physical interpretation of the ATR spectra. Namely, they are considered as frequency dependences of the Lambert absorption coefficient (alpha2) that are somewhat distorted due to anomalous dispersion. It is found that there are three reasons which cause differences between ATR and transmission spectra. The relative contribution of each of them is illustrated on the absorption band with a maximum at nu0 = 3400 cm-1, assigned to the stretching vibration of water molecules. The anomalous dispersion influence is also demonstrated on the benzene absorption band with a maximum at nu0 = 1035 cm-1.

The problem of non-ideal contact between an internal reflection element (IRE) and a rough sample under study is considered. The influence of the width of the gap on the intensity of attenuated total reflection (ATR) spectra is quantitatively analysed and equations for the absorbance (A) are derived. It is shown that in many instances surface roughness reduces A as compared with the true (i.e., corresponding to ideal contact between IRE and sample) value (Aid) by a factor of 2–5. Quantitative ATR analysis is evidently impossible under such circumstances without regard to the influence of the air gap. To solve this problem, two approaches are developed. Both methods are based on the measurement of the A values followed by calculation of Aid quantities. The first approach is based on the measurement of spectra with two different polarizations and the second utilizes the ratio of A values obtained for the band under study and for the “internal standard” band. It is shown that relative errors in Aid determination are less than 5% for both methods.

A method for quantitatively studying the absorption gradients in films of a thickness of an incident radiation wavelength, by means of attenuated total reflection (ATR) spectroscopy is proposed. The possibilities and limitations of the method are discussed.