Measurement of spectral line intensity of NO2 near 6.2 microns using a quantum cascade laser spectrometer

Recently, a joint research team from the Key Laboratory of Optoelectronic Information Acquisition and Processing of Anhui University, the Laboratory of Laser Spectroscopy and Sensing of Anhui University, and Ningbo Haier Xin Optoelectronic Technology Co., Ltd. published a paper titled "Measures of line strengths for NO2 near 6.2" μ Research paper on using a quantum cascade laser spectrometer.

Research Background
Nitrogen dioxide is a common pollutant, mainly derived from fossil fuel combustion emissions, natural lightning, and microbial processes in soil. NO2 in the atmosphere contributes to the formation of ground ozone, which may cause photochemical smog and increase the acidity of rainwater. Continuous exposure to high concentrations of NO2 may have various short-term and long-term adverse health effects on the respiratory systems of humans and animals. Therefore, developing a cost-effective and robust NO2 monitoring sensor system is crucial.

Many technical solutions have been developed for NO2 detection. Chemiluminescence and wet chemical analysis are commonly used for NO2 detection. However, these methods have a slow response time and low selectivity in distinguishing between NO and NO2, which limits their application. Optical methods based on absorption spectroscopy have high sensitivity, selectivity, and fast response, providing a powerful means for trace gas analysis. The laser absorption spectroscopy technology based on the mid infrared molecular fingerprint region is very ideal for trace gas analysis, as most atmospheric components have strong fundamental vibrational transitions in this spectral region, achieving high sensitivity and selective detection of trace gases. The commercially available continuous wave (CW) quantum cascade lasers (QCLs) in the mid infrared spectral region have been widely used in the development of spectroscopic techniques for quantitative analysis of NO2.

Experimental setup
In this work, we constructed a laser absorption spectrometer based on mid infrared CW-QCL in the laboratory to revise the spectral range from 1629 cm-1 to 1632 cm-1. The figure shows a schematic diagram of the spectral setup based on mid infrared CW-QCL for studying NO2 absorption spectral line parameters.

Ningbo Haier Xin Optoelectronic Technology Co., Ltd. provided a laser emitter (QC-qubeTM) and driver (QC750 TouchTM) for this project. A CW room temperature QCL chip is packaged in a thermoelectric (TE) cooled beam shaping package, driven by an integrated temperature and low noise current controller.

The laser source operates in the wavelength range of 1629 cm-1 to 1632 cm-1, without mode switching, and has an average output power of 30 mW. The laser frequency is scanned using triangular waves at a typical frequency of 100 Hz. The linewidth of the laser is approximately<10 MHz, so the broadening caused by the laser linetype can be ignored. The laser beam is initially collimated and passes through a sample cell with an optical path of 29.6 cm. The wedge-shaped CaF2 window placed at Brewster angle is used to avoid residual Etalon stripes. The QCL output beam is combined with visible red light (632.8 nm) through a ZnSe beam splitter to facilitate optical adjustment of the QCL output beam. The main beam passing through the sample pool is focused onto a TE cooled high-speed infrared photovoltaic detector through a convex lens, which can operate at room temperature. Therefore, the detector does not require liquid nitrogen refrigeration, simplifying the routine use of the system and allowing for long-term automated operation. The data is then obtained using a data acquisition board. The other part of the beam is coupled to an Etalon, which consists of two ZnSe mirrors with a free spectral range of 0.0163 cm-1.

Conclusion
In this study, we developed a compact spectral sensor based on thermoelectric cooling for the detection of trace amounts of NO2 using a room temperature continuous wave quantum cascade laser (RT CW-QCL). The high-resolution spectra of NO2 and N2 mixtures were studied in detail at room temperature (~296 K) and within a pressure range of 0-90 millibars. The absorption spectrum was fitted using standard Voigt profiles. Accurate measurements of line intensity and N2 pressure induced broadening coefficient were conducted for 43 NO2 spectral lines around 6.2 microns. This spectral region is very suitable for high-sensitivity detection of NO2 concentration. Our results are quite consistent with the latest HITRAN16 database in terms of spectral line intensity. Experimental spectral parameters will help upgrade our newly developed NO2 gas sensor system for atmospheric trace gas monitoring and industrial process control. In addition, we hope that these results have important value for the spectral database of NO2 molecules.

Source: Sohu