The Control of Errors in Infrared Spectrophotometry. VI. The Evaluation of Optical Constants by Combined Transmission and Attenuated Total Reflection Measurements

An improved technique is described for the determination of the optical constants of liquids in the infrared. It is based on a combination of transmission and attenuated total reflection (ATR) measurements. A novel application of the Kramers-Kronig transform function is involved whereby a single-valued integration constant is derived from a vector of refractive index measurements obtained by ATR. These are measured over a wavenumber range where the absorbance is low. It has been shown in earlier work that by an appropriate choice of a low refractive index contact material (in our case sodium chloride) the ATR method has high precision and accuracy under these conditions. The accuracy of the method is evaluated on the basis of a statistical treatment of the propagation of the estimated errors in the experimentally measured quantities, viz., the transmittance, the cell thickness, and the ATR measurements which establish the integration constant (anchor point) of the Kramers-Kronig transform function. The transmittance measurements are made at several cell thicknesses. The data reduction computer program, by which the optical constants are evaluated from the transmission measurements, monitors these sets of data and selects at each wavenumber the one having the minimal statistical error.