Rate-of-Growth Technique for the Measurement of Molecular Dipole Moments from Microwave Spectra at Weak Modulation Fields

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Abstract
Under suitable conditions, commonly met in practice, rotational lines observed with low amplitude zero‐based square‐wave Stark modulation have the shapes of the first or second derivatives of the zero‐field absorption lines. From the rate of growth of the peak‐to‐peak spectrometer deflection as a function of the modulation amplitude, the molecular electric dipole moment may be calculated. As this procedure does not require the resolution of individual Stark components, it should prove useful when the analysis of Stark splittings is not feasible, as in the case of cluttered spectra, spectra complicated by nuclear quadrupole hyperfine splittings, and spectra for which no low‐J transitions are accessible. Working equations are given for the case where the Stark shifts, when smaller than the linewidths, are linear or quadratic in the field. The method has been tried with OCS, CH3Br, and SO2. It appears that an accuracy of the order of several percent in the dipole moment can be achieved. Several experimental precautions and techniques are described, including a null method for measurement of the square wave voltage amplitude.