Quantitative millimetre wavelength spectrometry.: Part IV. Response curves for oxygen in carbon dioxide and nitrogen at 60 GHz Original Research Article

A millimetre wavelength (MMW) Fabry–Perot cavity spectrometer described in earlier work has been applied to the measurement of oxygen absorption at the 60 306.05 GHz transition, in gas matrices of air, nitrogen and carbon dioxide. The cavity was enclosed in a magnetic field impermeable Mumetal case to minimise the splitting of the spectral line by the magnetic field of the earth. Two depths of frequency modulation (FM), 240 and 600 kHz frequency deviation, at a rate of 1 kHz were applied to the microwave source. At the working pressure of 100 mTorr, the 240 kHz FM deviation gave the most intense absorption signal, yielding a limit of detection (signal: background=3) of around 1% pressure fraction of oxygen in either of the matrices. There was no matrix effect observed in passing from nitrogen to carbon dioxide as diluent. The response curves of absorption signal vs. fraction of oxygen in the mixtures was essentially linear at low concentrations, curving towards the abscissa at higher concentrations. This behaviour is not completely understood, but fitting the curves to response functions that reflected the characteristics of the absorption process, the cavity and electronic transfer functions and the effect of line broadening by matrix molecules proved to be a reasonable approach to modelling the instrument response. The pressure broadening coefficient of the oxygen line was measured in the range up to 1000 mTorr and found to be about 2.4 MHz mTorr−1, somewhat larger than the 1.89 MHz mTorr−1 reported in the literature for oxygen in the range 1–20 mTorr. An explanation for the discrepancy may involve some modulation broadening of the spectral line and possibly incomplete screening of the earthʹs magnetic field in the cavity. The MMW technique showed itself to be a useful approach to the monitoring of oxygen in gas mixtures of practical importance.