Raman spectroscopy is widely utilized in multi-component gas detection due to its rapid detection, good repeatability, and low cross-interference. However, the inherently low Raman scattering cross-section of gases results in exceedingly weak Raman signals, which impedes the further advancement of Raman spectroscopy gas detection. The multi-pass cavity-enhanced Raman spectroscopy technique can improve the excitation efficiency of Raman signals by converging the laser through multiple reflections to a focal point. Nonetheless, the employment of lenses to capture Raman signals at the convergence point results in limited collection efficiency. To address this challenge, Parabolic Mirror Cavity-Enhanced Raman Spectroscopy (PMCERS) is proposed aimed at concurrently enhancing the excitation and collection efficiency of Raman signal light. A parabolic mirror collection cavity is introduced to precisely couple the focal point of the parabolic mirror with the central convergence point of the multi-pass cavity, thereby collimating the Raman signal light at the focal point into parallel light emission. To enable efficient collimation of signal light emissions while preserving the integrity of signal collection efficiency, an annular reflector was integrated with the parabolic mirror. Through the analysis of the angles at which light rays were emitted from the focal point, the ideal dimensions of the annular reflector were ascertained. To verify the effectiveness of the PMCERS, comparative experiments demonstrated that, compared to traditional near-concentric cavity, PMCERS increased the Raman signal intensity detected in air by five times. In all, PMCERS enhanced both signal excitation and collection efficiency while simplifying the system structure, providing an efficient and stable solution for multi-component gas detection.
Aerosols consist of solid or liquid particles suspended in a gaseous medium. Distinct aerosol particles exhibit unique absorption and scattering spectra, thereby influencing the absorption and scattering of solar radiation. These processes contribute to warming the surrounding atmosphere and cooling the Earth's surface, subsequently impacting convective processes and cloud properties. Various types of aerosols are introduced into the atmosphere through anthropogenic and natural processes, exerting an influence on climate via direct or indirect radiative forcing. Accurately measuring aerosol optical properties, particularly absorption and scattering coefficients, holds both scientific and economic significance for assessing the climatic impacts arising from regional and long-term aerosol pollution characteristics. In this paper, photoacoustic spectroscopy technology integrating nephelometry technology had been employed for the simultaneous temporal and spatial measurement of aerosol absorption and scattering coefficients. A dedicated gas cell and detection system were constructed, with the design process outlined in detail. Experimental results illustrated that the combined utilization of the resonant photoacoustic cell and integrating sphere enabled the simultaneous detection of aerosol absorption and scattering coefficients. This underscored the potential applicability of the proposed method in analyzing aerosol optical properties, offering a solution for in-situ stereo-monitoring of atmospheric aerosols.
KEYWORDS: Photoacoustic spectroscopy, Signal detection, Thermal effects, Adsorption, Nitrogen dioxide, Photolysis, Signal to noise ratio, Light sources, Temperature metrology, Temperature control
Nitrogen dioxide (NO2) is a toxic gas to organisms, and one of the main factors in forming acid rain. Recently, some scholars have developed NO2 photoacoustic detection setups, but there are few reports about the effects of thermal decomposition and adsorption on photoacoustic detection of NO2. This work carried out detailed research on NO2 photoacoustic detection. Based on the photolysis effect of NO2, the thermal decomposition effect of NO2 excited by high-power laser was found and verified. Additionally, to reduce the influence of the adsorption effect of the photoacoustic cell wall on the detection results, a temperature control model of the photoacoustic cell was constructed, and the optimal detection temperature of 30 ℃ was ensured through experiments. The cross experimental verification was conducted with acetylene (C2H2) that was not decomposed by high temperature, which further explained the influence of NO2 thermal decomposition and adsorption effect on the detection accuracy. Based on the research results, a photoacoustic detection setup was built with a 450 nm laser and a differential H-type photoacoustic cell as the core. The experimental results showed that when the photoacoustic cell temperature was 30 ℃, the minimum detection limit of 206 ppt was achieved within 5 s average detection time. In conclusion, this work provides a reference for developing of high-precision NO2 photoacoustic detection setup.
Photoacoustic (PA) spectroscopy technology is widely used in the detection of various trace gases because it has some advantages such as zero background detection, strong reliability and high sensitivity. The capability of gas detection is directly affected by the performance of the PA cell as an important part of the PA detection device. A long optical path resonant sphere-tube coupled PA cell (STCell) was designed, which based on the spherical PA cell. The inner wall of the sphere was coated with diffuse reflection material as an integrating sphere, and the modulated light was reflected multiple times in the sphere to increase the optical path. In order to further amplify the PA signal, an acoustic resonance tube was coupled with the sphere. One-dimensional longitudinal resonance frequency of the tube was used to modulate light source, the maximum PA signal was got at the end of the tube away from the sphere. When the modulated light passed through the sphere, the structure was similar to the T-type PA cell (TCell). A PA experiment was carried out to compare the performance between TCell and STCell, and the gas to be measured was NO2. The PA signal amplitude of STCell was 10 times that of TCell when the concentration of NO2 was 10 ppm, and the limit of detection (LOD) (3σ) was 2ppb, which was reduced by 13.5 times. In this paper, long optical path with acoustic resonance was combined, which has reference significance for the design of trace gas PA cells.
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