The Absorption of Microwaves by Oxygen

Even though electrically non-polar, oxygen gas absorbs microwaves because the magnetic moment of the O₂ molecule interacts with electromagnetic fields. The resulting absorption is most pronounced, exceeding 10 db/km, for wave-lengths in the vicinity of cm, for then there is resonance to the spacings in the "rho-type triplet" or spin fine-structure in the ^3 ground state of O₂. There is also a subsidiary resonance near cm, and a non-resonant absorption at long wave-lengths due to diagonal matrix elements. The calculated values of the absorption are given in Table II and depend on the choice of the line-breadth constant which represents the effect of broadening by collision. Comparison is made of these theoretical results with the absorption in the cm region, observed by various experimentalists with different methods. It is concluded that 0. 02 cm^-1 is probably the best choice for c. The theoretical dependence of the absorption on pressure is discussed, and is particularly interesting because of the relation to the mechanism of collision-broadening and because the resonances to individual rotational lines are resolved at low pressures.