A Microwave Saturation Spectrometer for the Measurement of Linewidth and Absolute Intensity

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Abstract
This paper analyzes and describes a K‐band saturation effect spectrometer designed for the detection of resonance absorption in gas phase microwave spectroscopy. As a means of obtaining molecular information, this spectrometer exploits the nonlinear response of a balanced bridge to the amplitude modulation of a saturating microwave field. To enhance saturation the sample cell terminating the signal arm of the bridge is in the form of a high‐Q cavity consisting of a half‐symmetric confocal resonator excited in the fundamental TEM00q (q≃50) mode. Microwave detection is accomplished with a sensitive superheterodyne technique using phase coherent 30 MHz intermediate frequency (i.f.) conversion. By combining the double feature of in‐phase and quadrature‐phase i.f. demodulation, this system provides means for a fully automatic bridge balance giving rise to a detected molecular signal which depends on absorption only. The theory implied in the interpretation of this signal as a function of the unsaturated absorption coefficient of the gas is worked out and the techniques relevant to the construction and operation of the instrument are presented in detail. New and potentially useful methods of using the spectrometer for the precise determination of linewidth and absolute intensity of spectral lines are outlined. The capabilities of the methods have been tested on selected lines of the molecules OCS, C2H4O, and CH3OH and the resulting data have shown that a measurement accuracy on the order of a few percent can be achieved for both linewidth and intensity.