Abstract
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.
Highlights
Humidity plays a key role in photoacoustic spectroscopy (PAS) and sensing of many molecules
Signals usually increase with water concentration, especially in quartz-enhanced PAS (QEPAS), where modulation frequencies are approximately one order of magnitude higher than in conventional PAS, and fast relaxation is more important
Using the 2f -WM-QEPAS setup described in Section 2.1, the photoacoustic signal from 20 ppm of
Summary
Water is an efficient quencher that facilitates the conversion of the vibrational energy of other molecules into heat much more rapidly than other collision partners such as N2 or O2 typically would [1] This can be attributed mostly to the high polarity of water and the resulting attractive forces that occur during molecular collisions [2]. Many molecules, in particular small molecules at reduced pressure, relax too slowly to release all of the absorbed energy within one half period of photoacoustic modulation. Signals usually increase with water concentration, especially in quartz-enhanced PAS (QEPAS), where modulation frequencies are approximately one order of magnitude higher than in conventional PAS, and fast relaxation is more important.
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