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Photoacoustic Measurements of Lorentz Broadening in CO2 Between 25°C and 450 ° C*

Abstract : A variable temperature photoacoustic cell has been constructed and tested by studying the interplay of CO2, H/O, and NH3 in synthetic smoke. Saturation effects for CO2 and NH3 have been modeled and compared with experiments, and results are obtained for the vibrational relaxation rate associated with NH3-HzO collisions. The cell has been used for studying the temperature dependence of self-broadening and N2 broadening of CO2 lines. The temperature dependence of the scattering rate is well described by a T-n law with n = 0.77 in both cases. This result agrees with previous results obtained by tunable diode laser spectroscopy, but disagrees with results obtained by indirect methods. PACS: 33.70Jg, 34.50Ez, 7.65G The study of molecular absorption lines as a function of temperature is of interest from several points of view. The infrared vibration-rotation lines provide unique fingerprints of individual molecules, and may be used for monitoring purposes. The strength of the absorption can be used for quantitative measurements of molecular concentrations, and the shape and width of the lines provide information about the local environments, such as temperature and particle density. Thus, to fully extract all possible information from the line, it is necessary to know how parameters such as the line strength, and the collision cross section for various partners, depend on temperature. Applications which rely on such information are found in the realm of basic research, as well as in applied research. An example of the former is the study of planetary atmospheres, including that of the planet Earth, or conditions in interstellar molecular clouds, while industrial applications include the monitoring of molecules in exhaust from combustion processes. The most commonly employed method for studying temperature effects in infrared spectroscopy, is that of linear absorption. This works well for relatively strong lines, but for weak lines the absorption is seen as a small change in a large transmitted signal, and it is viewed on the background of the noise spectrum of the unattenuated *
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M Hammerich, L Vildrik-S~rensen, Hugo de Vries, J Henningsen. Photoacoustic Measurements of Lorentz Broadening in CO2 Between 25°C and 450 ° C*. Appl. Phys B, 1991, 53, pp.170 - 179. ⟨hal-02933797⟩