Microwave Absorption in Compressed Gases; Saturated Hydrocarbons

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Abstract
Sensitive resonant cavity techniques were used to search for dielectric loss in the lower members of the saturated hydrocarbon series, C1 through C5, at several frequencies in the range 2 to 24 kMc/sec. No detectable loss was found in methane (tanδ<8×10—7) or ethane (tanδ<4×10—7) up to the highest pressures examined, 69 and 36 atmos, respectively. Significant loss attributable to the presence of small permanent dipole moments was found in propane, isobutane, and isopentane. The results for isobutane, which show nonresonant or Debye type absorption characteristic of a symmetric top inversion spectrum having ``zero'' frequency, lead to an accurate evaluation of the dipole moment, namely 0.132 D. The effective collision diameters for impacts governing the Debye relaxation in both isobutane and isopentane are larger than the kinetic collision diameters.