THz cavity ring-down quantitative gas phase spectroscopy
Abstract
The development of cavity based infrared spectroscopy techniques has proved very successful, in particular cavity ring-down spectroscopy for sensitive measurements. The construction difficulty of high finesse cavities has hindered the application of this approach for THz frequencies. We have successfully demonstrated finesse values of 3500 by using a corrugated waveguide and photonic mirrors. This cavity which is 48 cm in length provides an interaction length of 1 km. A quantitative measurement of the absorption of a gas sample introduced into the cavity is made by THz Cavity Ring-Down Spectroscopy and applied to several gases and samples. Firstly, a pure gas that weakly absorbs the radiation at this frequency show that line strengths of 10−27 cm−1/(molecule.cm−2) may be measured. A sub-ppm trace of HCN a particularly polar molecule with a strong THz signature has been examined and demonstrates a Limit Of Quantification (LOQ) of 3 ppb. Industrial samples extracted from a waste recovery facility have been measured and compared with a certified gas analyser. The results for SO2, NO2 and NO have been compared using the different techniques and show a good agreement.
