Interferometric Determination of the Refractive Index of Liquids by Difference IR Spectroscopy

Abstract

A simple interferometric determination for the refractive index of liquids in the infrared is presented. The transmittance spectra of the liquid sample are recorded with two cells. One cell is assembled with a window material of high refractive index, producing interference fringes from multiple reflections (Fabry-Perot etalon); the other cell yields negligible interference as it is constructed from window material with a refractive index matched to that of the sample. By absorbance subtraction it is possible to eliminate most of the absorbance features, and the resulting channel spectrum can be used for the refractive index determination without the use of iterations and Kramers-Kronig transformation. The limitations are given by a theoretical consideration for Lorentzian band shapes, and two experimental examples are presented.