Quantitative millimetre wavelength spectrometry at pressures approaching atmospheric: Part I. Absolute determination of transition line strengths and sample concentrations with a confocal Fabry-Perot cavity spectrometer

The absolute determination of the millimetre wave power absorption coefficient of gas samples in a Fabry-Perot cavity whose resonant frequency is synchronised to a millimetre wavelength source is described. A theoretical treatment valid for pressures >200 Pa is developed. Absolute absorption coefficient measurements have been made on lines of SO₂ in the 50-60 GHz region that compared favourably with literature values. A calibration curve for SO₂ using the 59224.84 MHz line at 667 Pa over the concentration range 1-100% in N₂ has been produced. This technique could be especially useful for remote monitoring and process control applications because it is not necessary to scan the spectral line in order to determine the concentration of a species. The source runs at a single frequency coinciding with the peak of a millimetre wavelength (MMW) absorption line. This means that the technique could operate effectively at pressures up to and above atmospheric without the need for sophisticated MMW sources that can sweep across a wide frequency band.