Pressure and power correlated dependence in microwave spectroscopy using a non-linear-effect modulation spectrometer

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Abstract
The modification of the envelope of a modulated wave resulting from the non-linear absorption of a rotational transition can be used as the basic principle for the detection of transitions in gas phase spectroscopy. A spectrometer working on this principle is briefly described. Experimentally each transition contributes to two spectrometer signals, both of which have a non-Lorentzian lineshape. Nevertheless, the linewidth Delta nu can be easily correlated to the usual Lorentzian linewidth and is found to show the same pressure (P) and power ( pi ) dependence as in Stark modulation spectroscopy. Studies of pure OCS and of an OCS/air mixture have been carried out. In the case of pure OCS, the product S( Delta nu )4 is proportional to P3/2 and to pi 2. From the point of view of sensitivity the effective use of a non-linear-effect spectrometer is limited to cases where the sample pressure is intrinsically low.