Determination of Optical Constants in the Mid-Infrared Spectral Range by use of the Attenuated Total Reflection Method

Infrared (IR) spectroscopy is one of the most useful experimental methods in the investigation of hydrogen-bonded systems. However, application of transmission IR spectroscopy to aqueous systems is only of limited value due to the very strong water absorption. The necessity of very short pathlengths to obtain quantitative spectra is accompanied by very low reproducibility of the spectra obtained in such a way. However, in the last two decades, a reliable procedure which allows the calculation of optical constants [real n and imaginary k part of the complex refraction index n] from the spectra obtained by use of the attenuated total reflection (ATR) techniquepATR, has enabled a very accurate quantitative IR spectroscopy of liquid systems containing water.The use of calibrated infrared attenuated total reflection spectroscopy is discussed in the present work. The paper is organised as follows. After a short introduction, a theoretical clarification of processes corresponding to the attenuated total reflection is given. Here, an analysis of the processthrough Fresnel equations is followed by the discussion of the specific terms, such as effective number of reflections and penetration depth. In these terms, the difference between spectra obtained by transmission A and by ATR pATR was explained, specifying also the relation, through optical constants n and k, between these two forms of the IR spectra of the same system.The next section discusses the most reliable and up-to-date method for determining the optical constants of the ATR spectra, provided by Bertie and Lan (J. E. Bertie, Z. Lan, J. Chem. Phys. 105 (1996) 8502). This method calculates optical constants from s-polarised ATR spectra by a modified Kramers-Krönig transform of the reflectance Rs to the phase shift on reflection Θs. However, the method is developed only for the specific conditions of the ATR experiment, i. e. for CIRCLE cell, with a 45° incident angle and with equal intensities of s- and p-polarised light.Due to the different geometry of the ATR cell which is used in our laboratory (trigonal prism with an incident angle of 45°), the use of the method by Bertie and Lan to calculate optical constants from the ATR spectra obtained here is examined. From a calibrated single reflection spectra of water at 25 °C, obtained through a single-reflection Golden Gate ATR accessory, optical constants of water were calculated. The results are then compared by standard refraction and absorption index spectrum of water (J. E. Bertie, M. K. Ahmed, H. H. Eysel, J. Phys. Chem. 93 (1989) 2210). The difference between absorption index spectra k(a) was shown to be within the experimental error