Submillimetre- and millimetre-wave absorptions of some polar and non-polar liquids measured by fourier transform spectroscopy

Abstract

An assessment of the Fourier transform spectrometer indicates the advantages to be gained from its application to be the millimetre-wave region (10 to 2 cm^–1). The attainable resolving power is adequate for the study of broad absorptions such as occur in liquids below 100 cm^–1. With a mercury arc source, appropriate beam dividers, and an indium antimonide detector at 1 K, continuous refraction (n) and absorption (α) spectra have been obtained below 50 cm^–1(to 2 cm^–1 in some cases) for some typical polar and non-polar liquids. Results for water, aniline, 1,4-dioxan, cyclohexane, decahydronaphthalene, dimethyl acetylene (2-butyne) and 1-octyne are reported and considered in relation to earlier microwave dielectric data and far infra-red measurements. Water and aniline show strong, almost featureless, absorptions which are probably due to the superposition of three or four processes. The non-polar and weakly-polar liquids show much smaller absorptions : 1.4 dioxan has a profile consistent with a zero permanent electric dipole moment; 2-butyne and 1-octyne show α rising linearly with over the range 5-50 cm^–1. The origin of the absorption in non-polar liquids is different from the dominant mechanism in polar liquids although the peak values for both lie in the submillimetre-wave regions. The non-polar absorption peaks occur at frequencies which correlate with those calculated for molecular collisions in the liquids using the formulations of Mie and Bradley.