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Phase Optimized Photoacoustic Sensing of Gas Mixtures
Open AccessArticle

Anomalous Humidity Dependence in Photoacoustic Spectroscopy of CO Explained by Kinetic Cooling

Institute of Chemical Technologies and Analytics, Technische Universität Wien, 1060 Wien, Austria
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Appl. Sci. 2020, 10(3), 843; https://doi.org/10.3390/app10030843
Received: 30 December 2019 / Revised: 18 January 2020 / Accepted: 19 January 2020 / Published: 24 January 2020
(This article belongs to the Special Issue Quartz-Enhanced Photoacoustic and Photothermal Spectroscopy)
Water affects the amplitude of photoacoustic signals from many gas phase molecules. In quartz-enhanced photoacoustic (QEPAS) measurements of CO excited at the fundamental vibrational resonance of CO, the photoacoustic signal decreases with increasing humidity, reaches a pronounced minimum at ~0.19%V, and increases with humidity for higher water contents. This peculiar trend is explained by competing endothermal and exothermal pathways of the vibrational relaxation of CO in N2 and H2O. Near-resonant vibrational–vibrational transfer from CO to N2, whose vibrational frequency is 188 cm−1 higher than in CO, consumes thermal energy, yielding a kinetic cooling effect. In contrast, vibrational relaxation via H2O is fast and exothermal, and hence counteracts kinetic cooling, explaining the observed trend. A detailed kinetic model for collisional relaxation of CO in N2 and H2O is presented. Simulations using rate constants obtained from literature were performed and compared to humidity dependent QEPAS experiments at varying pressure. Agreement between the experiments and simulations confirmed the validity of the model. The kinetic model can be used to identify optimized experimental conditions for sensing CO and can be readily adapted to include further collision partners. View Full-Text
Keywords: photoacoustic spectroscopy; QEPAS; carbon monoxide; collisional energy transfer; kinetic cooling photoacoustic spectroscopy; QEPAS; carbon monoxide; collisional energy transfer; kinetic cooling
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Hayden, J.; Baumgartner, B.; Lendl, B. Anomalous Humidity Dependence in Photoacoustic Spectroscopy of CO Explained by Kinetic Cooling. Appl. Sci. 2020, 10, 843.

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