Collision and Saturation Broadening in Microwave Spectra

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Abstract
A theory of spectral line shape is presented for the case of a low pressure gas where only two energy levels (or several pairs of independent levels) are active in producing absorption. In Section I, an approximate set of differential equations is derived; these are solved in Section II. The method of approximation is shown to be consistent in Section III, and the final results are obtained in Section IV. A definite broadening of the lines in addition to that due to molecular collision is predicted as saturation is approached. The incident power needed for a given effect is proportional to the square of the pressure, and the maximum attenuation factor multiplied by the square of the half-width should be independent of incident power.