Impact Broadening of Microwave Spectra

Abstract

Pressure broadening in resonant and in nonresonant microwave spectra of gases, as well as the transition from resonant to nonresonant shape with increasing pressure, are treated in the impact approximation in a unified manner, based on the theories of Baranger, Kolb and Griem, and Fano. The relaxation rate in nonresonant absorption (Debye relaxation) is expressed in terms of collision cross sections; small deviations from a Debye shape are related to a second moment of the relaxation matrix, with illustrative application to C${\mathrm{H}}_{3}$Cl. A new formula is derived for the shape of resonance lines, as a function of frequency and pressure, which takes into account the special effect of collision-induced transitions between the upper and the lower levels of the line; this formula reproduces the shape of the inversion spectra of N${\mathrm{H}}_{3}$ and of N${\mathrm{D}}_{3}$.