Infrared Intensities of Liquids X: Accuracy of Current Methods of Obtaining Optical Constants from Multiple Attenuated Total Reflection Measurements Using the CIRCLE Cell

Abstract

The literature description of the Bertie-Eysel method for obtaining the optical constants (i.e., the real and imaginary refractive indices) of liquids from multiple attenuated total reflection measurements using the CIRCLE cell is brought up to date in this paper. The accuracy of the computation methods is explored by analyzing pATR spectra which are themselves calculated from known k(ν˜) spectra that contain single Lorentzian bands, and the corresponding known n(ν˜) spectra, and also from simulated, known, n(ν˜) and k(ν˜) spectra of pure liquid methanol and glacial acetic acid. The optical constants are recovered from the pATR spectra and compared with the known originals. It is shown that k(ν˜) spectra that contain k(ν˜) values up to 0.8, 0.7, and 0.6 can be obtained accurately when the real refractive indices are near 1.3, 1.4, and 1.5, respectively. The method is, thus, reliable for spectra that can be accurately measured from the multiple reflections in the CIRCLE cell. It is likely to be troublesome for higher values of the real and the imaginary refractive indices. However, these are best measured by single-reflection methods, and more direct ways of computing the optical constants are available for such methods.