Quantitative millimetre wave spectroscopy. part II: Determination of working conditions in an open Fabry-Perot cavity

Abstract

A confocal Fabry-Perot frequency modulated cavity spectrometer of quality factor 1.25 × 10 5 operating inside a chamber maintained at ambient temperature and pressure of 1 Pa to 1 KPa was employed for spectrometric measurements in the region of 72 GHz and 140 to 160 GHz. The spectrometer used a spatial filter to suppress unwanted, non-axial modes. The solid state microwave source frequency was derived from a phase-locked frequency synthesizer and detection was by a liquid helium cooled bolometer. Transitions in acrylonitrile, formaldehyde, and sulphur dioxide were studied demonstrating parts per million sensitivity for these species in atmospheric samples, whilst carbonyl sulphide samples were detected at sub-parts per million concentration. The effect of pressure on line intensities was studied in order to determine the optimum operating regime. It was found that the technique was not restricted to the 5–50 Pa region characteristic of centimetric wave spectroscopy, but was able also to function in the 0.1 to 1 KPa regime. Furthermore the intensities in this latter region were found to be not critically dependent on sample pressure. A treatment of the effect of pressure and depth of frequency modulation on absorption signals was carried out and the resulting theory applied to the observed intensity-pressure relationships. There was good quantitative agreement between the frequency modulation depth and cavity response characteristics and qualitative agreement between the pressure, frequency modulation and spectral line intensity characteristics. It became clear that the possibility of power saturation, coupled with the non-uniform power distribution within the cavity, was affecting the fits of theoretical curves to the observed data, and that taking this into account produced marked improvement in the fits. Nonetheless the treatment permitted some practically useful conclusions: at low modulation depths and pressures, the sharp spectral absorption peak makes identification of the target species easy, but extraction of quantitative information more difficult, as the intensity will depend critically on the power level in the cavity. At pressures in the 100 Pa region however, the signal obtained is maximal, power broadening minimal and comparative intensity measurements possible over a range of sample species and concentrations.