||Effect of Collisional Lifetime in Optoacoustic Detection of Pollutant Gases.
Christian, Wolfgang ;
||Allegheny Coll., Meadville, PA. Dept. of Physics.;Environmental Sciences Research Lab., Research Triangle Park, NC.
Air pollution detection ;
Gas analysis ;
Infrared spectroscopy ;
Molecular relaxation ;
Carbon dioxide ;
Nitrogen oxide(N2O) ;
Ultrasonic radiation ;
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The optoacoustic technique shows promise for pollution monitoring due to its small size and high sensitivity. This technique is fundamentally different from most spectroscopy in that absorbed energy is measured indirectly as a pressure change in the surrounding gas. Not all the absorbed energy is detected as a pressure change, the actual value depending on the collisional and thermal relaxation times. This research shows that relaxation effects in carbon dioxide begin to reduce the optoacoustic signal below 100 Torr. At 50 Torr the optoacoustic signal contains only half the absorbed energy. Collisional and thermal relaxation times of 7.5 micro sec and 0.1 sec are shown to correctly predict the decrease in the optoacoustic signal. A new calibration technique employing a piezoelectric crystal was developed for this research. The piezoelectric calibration was necessary because the microphone sensitivity varied by a factor of 3 as a function of total gas pressure. This technique is generally applicable in accounting for changes in microphone sensitivity.