Debye Relaxation in Symmetric‐Top—Foreign‐Gas Mixtures; Temperature Dependence of Collision Cross Sections

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Abstract
The Debye relaxation spectra of three symmetric‐top gases, CH3Cl, CHF3, and SO2F2, in the pure state and in dilute mixtures with the foreign gases, He, H2, D2, Ar, N2, CH4, CO2, and C3F8, were obtained over the temperature range −20° to 145°C at a frequency of 1220 MHz. Relaxation‐rate parameters and the corresponding collision cross sections are derived. The cross sections vary with the absolute temperature as T−m, where the value of m ranges from 0.3 to 0.9 for the different systems. The cross sections are considered from a classical kinetic veiwpoint and an empirical relation is obtained which accurately relates the dielectric cross section to the effective viscosity cross section (or the corresponding Lennard‐Jones parameters), the internal‐to‐orbital‐angular‐momentum ratio, and one additional parameter related in some way to the shape (prolateness or oblateness) of the top. Deviations in the shape of these spectra from the simple Debye form, which are attributable to a distribution of relaxation rates, are examined in some detail and correlated with the effective collision number.