Water-based enhancement of the resonant photoacoustic signal from methane-air samples excited at 3.3 μm

Photoacoustic spectroscopy is widely applied for trace-gas detection because of its sensitivity and low detection limit. In a previous work, where we studied the potential application to methane monitoring under a resonant excitation at 3.3 mu m, we showed that the signal from methane-nitrogen mixtures decreases with the addition of oxygen. This effect is due to an energy exchange between the nu (4) asymmetric stretching mode of methane and the first metastable level of oxygen. This process makes oxygen accumulate energy, thus hindering the generation of the photoacoustic signal. In this work, we study the possible addition of water, as a good collisional partner of oxygen, in order to obtain a greater sensitivity. We develop a model based on rate equations and find good agreement between theory and measurements. The experiment is carried out with a novel cell of rectangular cross section and a Q factor of 165 +/- 1. We find that 0.7 % water content is large enough to obtain a signal as high as in the methane-nitrogen case at atmospheric pressure.