Microwave Absorption in Methyl Halides

It has been confirmed that the absorption of 3-cm waves in methyl chloride and methyl bromide is entirely due to the inversion transition at zero wave number. In the J band the additional absorption observed has been assigned to the resultant effect of various rotational transitions of the molecule. In this connection it has been found that the following average values of the pressure-broadening parameter Δν give the best fit to the experimental results. Methyl chloride: Δν=0.5067 cm^-1/atmos (rotational line); Δν=0.124 cm^-1/atmos (inversion line). Methyl bromide: Δν=0.633 cm^-1/atmos (rotational line); Δν=0.122 cm^-1/atmos (inversion line). It is suggested that a more accurate fit can be obtained if the distribution of relaxation times for the inversion line and the shift of rotational lines to lower wave numbers is taken into account.Study of the temperature dependence of the absorption coefficient for methyl chloride in the 3-cm region indicates that, at low pressure, it varies as T^-3. This temperature variation of the absorption coefficient confirms that Δν∝1/T, as predicted by Anderson's or Margenau's theory of pressure broadening.