1.Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Hefei Institute of Physical Science, China Academy of Sciences, Hefei 230031, China 2.University of Science and Technology of China, Hefei 230026, China 3.Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130000, China
Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2016YFC0201100).
Received Date:05 December 2018
Accepted Date:22 January 2019
Available Online:01 April 2019
Published Online:20 April 2019
Abstract:Carbon disulfide (CS2) is a toxic volatile sulfur compound with flammability and harmfulness, which can seriously harm the human health and threaten the industrial production safety. Therefore, it is of high importance for monitoring CS2 concentration in the air. Tunable diode laser absorption spectroscopy is very suitable for the detection of trace gas for it possesses high sensitivity and fast response. And the precise knowledge of spectroscopic parameters is essential for deducing the CS2 concentration. However, primary database including HITRAN and GEISA lacks spectroscopic parameters of CS2. Thus, to address this issue, a measurement system of absorption spectrum is built for determining spectroscopic parameters by using a quantum cascade laser with narrow linewidth and high output power operating near 4.6 um as a light source. In this paper, direct absorption spectroscopy is used to measure the CS2 absorption spectra under different sample pressures and the environment temperature is controlled at 296 K, which is adjusted by an air conditioner. We intensively study the absorption spectra of CS2 in a range between 2178.99 and 2180.79 cm–1.According to the relevant reports and the need of actual measurement, four absorption lines are mainly measured in a range of 2180.5?2180.74 cm–1. Combining with the multiple linear regression algorithm based on the nonlinear least-square method and Beer-Lambert law, the integrated area and Lorentz line width of measured CS2 absorption spectrum can be determined. Then the spectroscopic parameters including absorption line intensity and air broadening coefficient are precisely obtained by linearly fitting the integrated areas and Lorentz line widths of CS2 absorption spectra at different pressures. Moreover, nitrous oxide (N2O) absorption spectrum with high spectral resolution is measured to calibrate the central position of carbon disulfide absorption line according to its known line position extracted from HITRAN database and the results obtained by etalon. The calculated results show that the uncertainty of line intensity and air broadening coefficient are less than 5% and 15%, respectively. It demonstrates that the measured spectroscopic parameters of four absorption lines for this study can be recorded in the database of HITRAN, which is very important for trace gas sensing of CS2. In the future, we will further improve the system for measuring CS2 absorption line parameters to fill in the gaps in their spectral parameters in HITRAN and GEISA databases. Keywords:carbon disulfide (CS2)/ quantum cascade laser/ tunable diode laser absorption spectroscopy/ spectroscopic parameters
由于数据库中并无CS2的参数, 因此本文通过测量同一扫描范围内N2O的高分辨吸收光谱, 结合数据库中N2O已知谱线的中心波长等参数以及同步测量的锗标准具干涉信号, 对未知CS2吸收谱线的中心波长进行精确标定. N2O吸收光谱和锗标准具干涉信号测量结果如图3所示. 实验中, N2O的浓度为3988 ppm, N2O压力控制在1968 Pa. 激光器在周期性起始时间段内处于出光阈值以下, 以此作为背景光辐射、电子学噪声和探测器等造成直流偏移量的本底参考[20, 21]. 图 3 N2O吸收光谱信号和标准具干涉条纹 Figure3. The measured N2O absorption spectrum signal and obtained interference fringes by optical etalon.
本文采用基于非线性最小二乘的多项式拟合算法对无吸收部分信号进行拟合, 获得激光器的出光背景, 得到CS2吸收光谱信号的归一化背景. 不同压力下纯样品浓度的CS2吸收光谱信号如图4所示. 通过多元线性回归算法并结合Voigt线型函数对以上吸收信号进行拟合, 得到的残差整体小于0.06 (图4). 图5为对CS2四条吸收谱线在不同压力下的积分吸光度进行线性拟合的结果, 线性相关系数分别为0.98768 (2180.54844 cm–1), 0.99859 (2180.55578 cm–1), 0.99716 (2180.65676 cm–1), 0.99736 (2180.69443 cm–1). 根据(3)式得到了温度为296 K条件下四条CS2的谱线线强参数(2180.54844 cm–1, 2180.55578 cm–1, 2180.65676 cm–1, 2180.69443 cm–1), 记录在表1中. 图 4 2180.65676, 2180.69443, 2180.54844和2180.55578 cm–1处CS2 Voigt线性拟合结果及残差 Figure4. Voigt fitted line and residual of the CS2 spectrum at 2180.65676, 2180.69443, 2180.54844, and 2180.55578 cm–1, respectively.
图 5 2180.65676, 2180.69443, 2180.54844和2180.55578 cm–1处CS2 线强线性拟合结果 Figure5. The linear fittd results of line-strength of the CS2 at 2180.65676, 2180.69443, 2180.54844, and 2180.55578 cm–1.
${\rm{\nu }_0}/{\rm{c}}{{\rm{m}}^{ - 1}}$
S(T0)/cm·molecule–1
Uncertainty/%
Air broadening/cm–1·atm–1
Uncertainty/%
2180.54844
9.19 × 10–22
1.792
0.087
0.336
2180.55578
1.97 × 10–21
4.055
0.092
3.384
2180.65676
2.24 × 10–21
4.537
0.103
7.098
2180.69443
1.11 × 10–21
2.232
0.086
14.687
表1计算得到线强值、不确定度以及空气展宽值和不确定度 Table1.The calculated spectroscopic parameters including line-strengths, air broadening coefficients, and the corresponding uncertainty.
空气中的CS2浓度大约在ppb量级, 结合(4)式, 在实际测量中自展宽对光谱信号线宽所起的作用可以忽略不计, 因此本文主要对CS2的空气展宽系数进行了测量. 实验中, 首先用高纯氮气($\geqslant $99.999%)多次冲洗气体吸收池, 随后用空气稀释至一个大气压, 使得CS2的体积分数只占到1%, 再对吸收池进行抽气得到CS2的混合气体吸收光谱(图6). 对吸收光谱信号选用标准Vogit线性函数进行拟合, 计算得到对应压力下洛伦兹展宽半峰全宽值, 由(5)式可知在室温温度(296 K), 对不同压力下的洛伦兹展宽值进行线性拟合即可获得空气展宽系数值(图 7). 图 6 2180.65676, 2180.69443, 2180.54844和2180.55578 cm–1处含1% CS2的混合气体的拟合结果 Figure6. The Voigt fitted results of mixed gas containing 1% CS2 spectrum at 2180.65676, 2180.69443, 2180.54844, and 2180.55578 cm–1, respectively.
图 7 2180.65676, 2180.69443, 2180.54844和2180.55578 cm–1处不同压力的洛伦兹线宽 Figure7. Linear fit of Lorenz linewidth for different pressures at 2180.65676, 2180.69443, 2180.54844, and 2180.55578 cm–1, respectively.