Abstract
The optical absorption in the region 20–100 cm−1 has been measured by Fourier transform spectroscopy for the pure liquids of the ortho and meta isomers of difluoro‐, dichloro‐, and dibromobenzene at a temperature of 20°C. The absorptions are very broad having half‐widths of about 80 cm−1 and the peak value of the absorption coefficient is higher than that predicted by the Debye theory of dielectric relaxation for all the substances studied. The theories proposed for absorption in the far‐infrared region are inadequate and do not explain the experimental results. It is concluded that the strong absorption is predominantly due to the rotational transitions of the molecule and that a similar phenomenon should occur in all polar liquids.

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