Theory of Microwave Nonresonant Absorption and Relaxation in Gases

Abstract

A quantum derivation is given of nonresonant absorption and dielectric and magnetic relaxation in gases with molecules which have diagonal electric or magnetic dipole matrix elements. General expressions for the shape and width of pressure-broadened nonresonant lines are obtained by solving a master equation for the polarization. Relaxation by bimolecular collisions which change the magnetic (orientational) and total-angular-momentum quantum numbers of the dipoles is treated in the impact approximation. The case of weak collisions which change these quantum numbers by one or two units is solved in detail; intermediate and strong collisions which cause much larger quantum changes are considered qualitatively. Formulas are derived for computing collision cross sections from known intermolecular interactions for weak collisions. On comparing experimental results for symmetric-top gases with the theory, it is concluded that strong collisions are responsible for the relaxation process.