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Dispersion studies of the 22 GHz water vapor line shape II. Instrumental correction

Journal of Quantitative Spectroscopy and Radiative Transfer
Publication Date
DOI: 10.1016/0022-4073(71)90023-9


Abstract Anomalies in the resonance dispersion of the pressure-broadened water vapor line at υ 0 = 22.235 GHz, which were reported in Part I, (1) are resolved. The pressure-scanning differential-refraction spectrometer (microwave dispersometer) gives rise to a signal enhancement as the line center moves within the width of the dispersion discriminator which is a dual-mode transmission cavity. Resonance absorption prevents the frequency of peak transmitted power from coinciding with the resonance condition of zero phase, thus systematically distorting the molecular resonance dispersion. An expression, valid for | υ − υ 0| greater than the cavity half width, is derived which predicts apparent departure from Lorentzian behavior. The results reported in I as well as results for the 23.6 GHz rotational 3 1,2 → 3 2,1 line of C 2H 4O (ethylene oxide) show a Lorentzian molecular line shape. At pressures above 1 torr nonresonant polarization effects become detectable. Instrumental distortion, and Doppler-, wall-, and saturation-broadening effects at low pressures (<50 mtorr) are accounted for only qualitatively to determine the resolving power and to define v 0.

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