| Hollow waveguides(HWGs)have been widely used in spectroscopic gas sensing devices as both optical waveguides and gas transmission cells,providing an extended optical path length in a small volume,yielding high-sensitivity measurements.However,the structure of HWG lead to be more susceptible to the problem of gas adsorption,especially for polar molecules such as ammonia.In this thesis,the dynamic process of gas adsorption and desor:ption in an Ag/AgI-HWG was quantitatively analyzed by tunable laser absorption spectroscopy(TLAS).The influence of adsorption effect on spectral measurement under different factors was studied.The main works and innovations of the thesis are summarized as follows:Firstly,an approach to the simultaneous measurement of gas parameters by a single absorption line was realized.Based on degraded deconvolution of the Voigt profile by the empirical analytical approximation,we simultaneous inversed the gas concentration,temperature and pressure from a single absorption line.This approach improved the analytical efficiency of Voigt profile.This practical approach was demonstrated by theory analysis,spectral line selection criteria and noise evaluation,which laid the foundation for the adsorption measurement of ammonia in HWG.Secondly,a system with high time resolution and high precision was designed and verified for ammonia measurement.According to the requirements of ammonia concentration detection and spectral measurement accuracy,based on QCL and HWQ the optical system with reference cell was constructed.The accuracy of the measurement system was analyzed and optimized.The reliability and accuracy of the measurement system were verified under laboratory conditions based direct absorption spectroscopy.The results show that the detection sensitivity was less than 1 ppm when the time resolution was 1 ms,which meet the demand of adsorption analysis.Thirdly,the dynamic process of NH3 adsorption and desorption in an Ag/Agl-HWG was quantitatively analyzed.Based on high precision TLAS system,the effects of pressure and temperature on the adsorption were investigated by determining the number of adsorbate molecules under different conditions.The density of NH3 adsorption was of the order of about 1015 molecules/cm2 and the corresponding F value was about 5.89.The effects of pressure and temperature on adsorption were investigated by isothermal experiments and isobaric experiments.The adsorption capacity increased with pressure and decreased with temperature,which is consistent with Le Chatelier's principle.The adsorption kinetic process also fit well to both the pseudo-first-order and pseudo-second-order kinetic model.In addition,by compared the absorption spectra of ammonia in the reference cell with those in the HWG,the red shift of absorption spectra in the process of adsorption was determined,and the offset was about 10-2?10-3cm-1.In conclusion,based on mid-infrared TLAS,the adsorption on HWG was studied quantitatively for the first time to the best of our knowledge.In this thesis,the amount of adsorption and the wavelength shift caused by adsorption are obtained.This study has a significant reference value for accurate measurement of gas concentration based on HWG and the adsorption mechanism of ammonia on the surface of HWG. |
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