Intensity Law for Microwave Spectroscopy: Theoretical and Experimental

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Abstract
A method is described for extracting accurate molecular information from pure‐rotational absorption‐line‐intensity data. This method can be applied successfully to the relative intensity of two or more lines even when these lines occur at widely separated frequencies. It is shown that the experimental function, log spectrometer signal amplitude at the line frequency vs log corresponding incident microwave‐power level can be superimposed on a theoretical intensity function called the intensity law by a translation of the origin. This origin displacement is related to the molecular concentration of the absorbing species and the rotational broadening–relaxation time associated with the particular transition. The intensity law is independent of the line frequency or sample pressure and depends only on the microwave power density distribution in the sample. This distribution varies as function of frequency in the Stark cell due mainly to waveguide wall losses. Neglect of this variation can result in large errors in molecular quantities calculated from relative intensity data.