Far infra-red absorptions of non-dipolar liquids. Part 1.—Experimental measurements, quadrupole interpretation and model calculations

The far infra-red absorption spectra of a number of non-dipolar liquids have been obtained using the NPL-Grubb Parsons cube interferometer, adapted to employ phase modulation. Marked improvement in the precision and reproducibility of the absorption measurements has been achieved. The characteristic absorption of non-dipolar liquids in the far infra-red has then been evaluated in terms of an “effective” quadrupole moment of the molecules involved. The quadrupole moments so calculated agree with available literature values, thus supporting the view that the submillimetre-wavelength absorption of non-dipolar liquids largely has its origins in a quadrupole-induced dipole appearing in the molecules.Model calculations show there is adequate “dipole strength” induced by the quadrupole field of a neighbouring non-dipolar molecule to account for the absorptions observed. The approximations involved in point multipole representations of adjacent-molecule interactions are estimated by comparison with the better approximations of local-charge models.