Knowledge Bank

The utilization of the saturation effect $spectrometer1$ for the determination of the line breadth and the intensity of microwave absorption lines is described. As a means of detection this spectrometer makes use of the molecular response of a balanced bridge to the power modulation of a saturating microwave field. In this paper a new method of measuring peak intensity is presented which overcomes most of the difficulties common to this type of measurement. The method is based on an amplitude comparison process between the molecular sideband output of the spectrometer and a phase quadrature signal representing the portion of the sideband input which is not discriminated against by the sharp frequency selectivity of the bridge. In this way the peak absorption is obtained directly in terms of the modulation frequency of the source and a saturation factor determined by the geometry of the resonant gas cell. The capabilities of the method have been tested on selected lines of $OCS,C2H4O$ and $CH3OH$ for which precise line width measurements are also reported. The power saturation of several lines of these molecules have been investigated with the spectrometer and the observed results confirm the essential features of the Karplus-Schwinger theory of saturation, indicating in particular that the collisions effective in broadening the lines are also effective in bringing about thermal equilibrium of the populations. However an anomalous shift is observed in the saturation curves of $CH3OH$.