Collision induced microwave absorption is reported in pure CO2, and CO2-Ar, CO2-CH4 mixtures in the 70 G H z (2.3 cm-1) region at a temperature of 22°C, using a sensitive cavity technique previously described. The results in pure CO2 in the very low density region from 5 to 30 amagat accurately establish the dependence of the loss on the square and cube of the density, and the relaxation times are calculated. The experimental results agree well with previously reported lower frequency data at 0.3-0.8 cm-1 which establishes the linear dependence on frequency of the absorption up to 2.3 cm-1. There is also good agreement with an extrapolation of higher frequency infrared results of Ho et al. The relaxation times associated with the two and three body collisions are shown to be nearly equal at room temperatures with T~ = 0.84 x 10-12 s and T~ = 1.0 x 10-12 S. Higher order dependence on the density is observed for the CO2-Ar and CO2-CH4 mixtures. The results are compared with earlier low frequency measurements at 0.8 cm-1 and with the theory of Maryott and Kryder, taking account of correction terms in the dielectric virial coefficient according to Bose and Cole.
Accurate measurements of collisionЃ]induced absorption in CO2 are made at a number of temperatures in the range from − 40 to 60°C in the wavelength region 7–250 cm−1. Direct evidence for the separation of the pure translational band from the rotational–translational band is obtained at all temperatures. This and other aspects of the band shape are discussed. Over the entire temperature range, the experimentally determined Kramers–Kronig integral is found to be in good agreement with the theoretical value, i.e., the static dielectric constant. This agreement is achieved only when the contribution of the quadrupole–quadrupole energy in the radial distribution function, of particular importance for CO2 because of its large quadrupole moment, is calculated accurately. A value of the quadrupole moment is obtained, (4.5 ± 0.2)10−26 esu, which is in satisfactory agreement with that obtained by the method of Buckingham and Disch, which does not depend on a knowledge of intermolecular force constants. Induction due to higher multipole moments and the overlap interaction is considered.
