The dielectric properties of simple polar liquids at millimetre wavelengths
Previous methods of determining the complex permittivity, e* = e' - je", at millimetre wavelengths are reviewed, and at a wavelength of 0.848cms six different ways of determining this quantity from the experimental observations are compared. It is found that the method first described by Fatuzzo and Mason is the most suitable for determining the temperature variation of the complex permittivity at wavelengths of and 0.435cms, and the three sets of apparatus are adapted to cut down experimental errors to a minimum. The complex permittivity, for several simple liquids, is determined at each of the three wavelengths over a temperature range from 50aC down to the freezing point of the liquid The static permittivity Es and the refractive index at the Sodium-D line nD are also measured over this same temperature range. Assuming that the dispersion of E* is given by Debye's equations, then, knowing the static permittivity and the experimental value of E* at one other wavelength, the permittivity at the high frequency end of the dispersion curve E-infinity and the critical wavelength lambdac can "be calculated. Using the value of E* at each of the three experimental wavelengths in turn, three different values for E-infinity and lambdac at each temperature are calculated. The value of E-infinity and lambdac at 3.332cms are taken as the correct values, and the deviation of the values at the other two wavelengths from this value noted. This deviation is to smaller values of E-infinity and lambdac as the wavelength shortens and the deviation increases with decreasing 'temperature. This type of deviation is explained by the presence of a second absorption band centered at sub-millimetre wavelengths. By considering the amount of the deviation and the difference E-infinity - N&phis;2 as compared with ES - N&phis;2 it is concluded that; the second absorption band is due to a resonance absorption, rather than a Debye type absorption, and having a resonant frequency which decreases slightly with increasing temperature.