High Precision Measurement of Spectroscopic Parameters of CO at 2. 3 m Based on Wavelength Modulation-Direct Absorption Spectroscopy

Wavelength modulation-direct absorption spectroscopy (WM-DAS) combines the advantages of DAS and WMS, which can directly measure the absorbance and improve the measurement signal-to-noise ratio (SNR). It can be used to measure the spectroscopic parameters of gas molecular spectral lines. Firstly, the WM-DAS method is used to measure the absorbance of CO molecule 4 300. 700 cm(-1) spectral lines under the condition of 24. 151 mu mol center dot L-1 CO concentration, room temperature and pressure, combined with Herriott cell with an effective optical path length of about 45 m. The absorbance is optimally fitted by Voigt profile (VP)and the results show that the standard deviation of the absorbance fitting residual from WM-DAS is reduced by more than half compared with that from the traditional DAS method, which proves that the anti-interference ability of the WM-DAS method is stronger than that of the DAS method. Then, this method combined with an absorption cell with an optical path of about 50 cm was used to measurethe absorbance of 8 weak absorption spectral lines of CO at 4 278 similar to 4 304 cm(-1) under different pressures. The CO standard gas with a concentration of 0. 411 mu mol center dot L-1 was used in the experiment. The measured absorbance was fitted by VP, Rautionprofile (RP) and quadratic-speed-dependent-Voigt profile (qSDVP) to obtain the collision broadening coefficient gamma(0) (T-0), the Dicke narrowing coefficient beta(0) (T-0) and the speed-dependent collision broadening coefficient gamma(2) (T-0) in qSDVP, respectively, and the uncertainty of the measurement results was analyzed. The measured gamma(0) (T-0) obtained by Voigt profile fitting agree well with those from the HITRAN database, with a relative error of less than 1%. The measurement results of beta(0) (T-0) and gamma(2) (T-0) provide an important data for further perfecting molecular spectral database and high-precision measurement of gas parameters.