The laboratory spectroscopy of true line shape in MM/SubMM range

The report deals with precision measurement of spectroscopic parameters, which can be extracted from the shape of observed molecular lines. Two spectrometers in their modem versions are described. Both spectrometers are based on MM/SubMM backward-wave oscillator (BWO) tubes phase-locked against harmonic of frequency standard. The first one is automated spectrometer with acoustical detection (RAD) of radiation absorption operating in pressure range from hundreds of milliTorr to ten Torr. The second one is the resonator spectrometer with fast digital back and forth frequency scanning operating in a pressure range up to atmosphere. Use of various line shape models for experimentally observed collision-broadened lines in mm/submm wavelength range is demonstrated. Sources of line parameters determination errors such as wrong line shape model selection, improper accounting for baseline or apparatus function, limiting range of line record are analyzed. Less than 10 kHz measurement accuracy of rather weak oxygen lines central frequencies and 10 kHz/Torr accuracy of the line pressure broadening parameters determination, ∼1% measurement accuracy of 183-GHz water line integral intensity, first experimental measurement of a small mixing parameter for 118 GHz oxygen line are achieved in our experiments as result of treatment of observed with high signal-to-noise (S/N) ratio shapes of spectral lines and careful accounting for the line shape distortion factors.