[Zurück]

J. Hayden, B. Baumgartner, B. Lendl:
"Anomalous humidity dependence in photoacoustic spectroscopy of CO explained by kinetic cooling";
Applied Sciences, 10 (2020), 3; S. 843.

Water a ects 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 e ect. 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 partner

photoacoustic spectroscopy; QEPAS; carbon monoxide; collisional energy transfer; kinetic cooling

http://dx.doi.org/10.3390/app10030843