Microwave Absorption in Compressed Carbon Monoxide and Nitrous Oxide

Abstract

The absorption of microwaves by both CO and N2O has been measured at a frequency of 9·75 GHz using a resonant cavity technique. The power absorption coefficients were measured at pressures up to 670 bars for CO at 20·5°C, 54 bars for 20 at 25°C, and 121 bars for N2O at 40°C. In all cases the power absorption coefficient was found to increase with density up to the highest pressures reached. The theory of Van Vleck and Weisskopf fails to account for the measured absorption except at low densities. At higher densities the measured absorption was in excess of that predicted by this theory and the discrepancy increased with increasing density. The theory of Ben-Reuven accounts for the absorption up to intermediate densities but at high densities it also predicts values which are smaller than the measured ones. However, it is possible to account for the measured absorption by considering it to be the sum of a resonant contribution, which is assumed to be given by the Van Vleck-Weisskopf theory, and a nonresonant collision-induced contribution which increases with density and is assumed to be the result of collisions so strong that rotation of the molecules is quenched.