Optical measurement of gas vibrational-translational relaxation time with high accuracy by the laser photo-acoustic set-up

Abstract

This study aimed to measure the vibrational-translational (V-T) relaxation time of gases using photo-acoustic technique with high accuracy. For this purpose, a cylindrical resonator and its acoustic filters were simulated and then fabricated. The results showed that minimum detectable concentration of Ozone and Sulfur hexafluoride by this set-up was measured 7 and 33 ppb, respectively. So, this optical set-up can be used as a highly sensitive sensor to detect gases. Then, the signal and the resonant frequency changes in the presence of some noble gases in different pressures, at 0.42 W laser were studied. Finally, the V-T relaxation time of these gases at different pressures (100–800 Torr) of buffer gases was measured in the order of milliseconds. The V-T relaxation time of Ozone and Sulfur hexafluoride at 100 Torr Helium buffer gas was obtained 3.7 and 3.1 ms, respectively. Also, for both ozone and sulfur hexafluoride in the presence of helium buffer gas, the lowest V-T relaxation time and photo-acoustic signal and the highest frequency were obtained.