An experimental study of the shapes of microwave spectral lines in gases at low pressures

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Abstract
A spectrometer which has been used for studying the shapes of microwave spectral lines in gases is described. Radiation from a klystron was passed through a waveguide cell and detected with a crystal diode. The klystron frequency was modulated sinusoidally with a frequency of f/n, n being integral and in the range 1 to 5, and the detected output at frequency f was observed on an oscilloscope. In this way the first five harmonics of the modulation frequency were observed. The mean klystron frequency was swept slowly across the spectral line and the frequency separations between the various maxima and minima of the harmonic outputs were measured using frequency markers. The results were compared with those deduced from three theoretical lineshape models. Three cases were studied; the self broadened (J,K)=(6,6) line in the inversion spectrum of NH3 the same line broadened by He; and the self broadened J=1 to 2 line in the rotational spectrum of COS. In all cases the lineshape was represented to within the experimental error by a Lorentz shape corrected for the Doppler contribution.