Effects of H2O and SF6 on CO Molecular Relaxation in a Cantilever-Enhanced Fiber-Optic Photoacoustic Sensor

Abstract

The effects of H₂O and SF₆ on the relaxation rate of CO molecules are studied using a fiber-optic photoacoustic sensor (FOPAS) that integrates a resonant multipass photoacoustic cell with a fiber cantilever. The CO photoacoustic signal is enhanced by approximately 1 order of magnitude under the induction of H₂O. SF₆ has a high density compared to N₂, which causes the photoacoustic resonance frequency to shift to low frequency. The photoacoustic responses of CO/SF₆ and CO/N₂ at low frequency are tested with and without H₂O to verify that SF₆ gas has a promoting effect on the relaxation process. The impact of the increasing H₂O concentration on the CO/N₂ photoacoustic signal manifests in three distinct stages, which are reflected in gradual increase, sharp increase, and eventual saturation. The corresponding H₂O concentration ranges of the three different stages are 2000–12,000, 12,000–18,000, and above 18,000 ppm, respectively. The SF₆ ratio in CO/SF₆ and CO/N₂ gas is precisely controlled, and the influence mechanism of the background gas mixture changes on the resonance frequency, photoacoustic response, and Q-factor is studied. Furthermore, the responsivity and noise of the fiber-optic photoacoustic CO sensor are evaluated under the induction of H₂O and SF₆. The detection limits of CO/N₂ and CO/SF₆ are 10 ppb with averaging times of 100 and 400 s, which makes this study applicable to the detection environment with N₂, SF_6, and N₂–SF₆ mixtures as background gas.