Microwave Absorption in Ethyl Chloride

The variation of the microwave absorption coefficient with pressure in ethyl chloride has been determined at 7392 Mc/sec and 8780 Mc/sec. The values at one atmosphere pressure and temperature 300°K are 4.9×${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and 6.9×${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, respectively. From the known values of the rotational constants of the molecule, all the allowed rotational frequencies of the $P$, $Q$, and $R$ branches for all significant $J$ values have been computed; the theoretical intensities of these lines have been computed approximately with the help of the transition intensities tabulated by Cross, Hainer, and King. The formula of Van Vleck and Weisskopf has been used to calculate the absorption due to these lines, and it is found that practically all the absorption observed in the 3 cm and 1.25 cm regions is due to the low-frequency $Q$-branch rotational lines, most of which are practically at zero frequency because the asymmetry in the molecule is very small. The value of the pressure-broadening parameter which gives the best fit in the experimental data at all pressures up to one atmosphere is 0.25 ${\mathrm{cm}}^{-1\mathrm{}}$ per atmosphere. The observed absorption is thus equivalent to a Debye-type nonresonant absorption due to a zero-frequency line of intensity 22.83×${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$/atmos and $\Delta \nu =0.25\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$/atmos.