SEPA United States Environmental Protection Agency Environmental Sciences Research Laboratory Research Triangle Park NC 27711 A * Research and Development EPA-600/S7-81-026 May 1981 Project Summary Effect of Collisions! Lifetime in Optoacoustic Detection of Pollutant Gases William McClenny The optoacoustic technique shows promise for pollution monitoring due to its small size and high sensitivity. This technique is fundamentally dif- ferent 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 colli- sional 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 relaxa- tion times of 7.5 m sec and 0.1 m sec are shown to correctly predict the decrease in the optoacoustic signal. A new calibration technique em- ploying a piezoelectric crystal was developed for this research. The piezo- electric 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. This Project Summary was developed by EPA's Environmental Sciences Re- search Laboratory, Research Triangle Park, NC, to announce key findings of the research project that is fully docu- mented in a separate report of the same title (see Project Report ordering information at back). Introduction The optoacoustic signal resulting from absorption of radiation by a con- fined gaseous absorber depends on the •collisional relaxation time, tc, of the absorber. Existing mathematical models of the physical processes involved in the generation of the photoacoustic signal, S, predict the dependence of S on tc as well as other system parameters such as t, and tt, the radiative relaxation time and thermal relaxation time, respective- ly. Experimental documentation of cor- responding values of S and the pressure in an optoacoustic cell can be used to infer values of the relaxation times. The technique to be discussed is generally applicable to characterizing optoacoustic response for specific gas mixtures. Results of characterization can be used in optimizing system response with respect to detection of trace gases in air. Results A COa laser was used as a source of radiation to illuminate a gaseous ab- sorber contained in a cylindrical cell. An electret microphone was mounted at the cell wall. Modulation of the laser radiation by a mechanical chopper resulted in small pressure changes that were recorded as a function of the total pressure in the cell. Microphone sensitivity was noted to vary as the total cell pressure was changed over the range of 10 to 760 torr. The optoacoustic cell was calibrated i US. GOVERNMENT PRINTING OFFICE-1861 -757-012/7111 ------- by the use of a piezoelectric crystal attached to the cell wall. Changes in crystal length in response to an applied electric field caused precise volume and, hence, pressure changes in the cell. Calibration data depended on the frequency of modulation of the laser radiation and on the cell pressure. The main experiment consisted of recording the corrected microphone response as a function of total pressure. This data was then compared to a mathematical model for optoacoustic response and values of tc, t, and tt were predicted. The model expression was taken from the work of L Rosengren as published in Infrared Physics, 13, 173 {1973). Rosengren's expression was used to fit the experimental data. The relaxation times were determined from the experimental data using a standard chi-squared minimization program. For pure COz, the C02 laser radiation at 10.6 microns was used to obtain the values: tc = 7.5 microsec; tr = 0.002 sec and tt = 0.11 sec. The thermal relaxation time tt was determined to be 0.10 ± 10% sec by an alternative technique. Values of tc published by other experimenters ranged from 7.5 to 12.0 microsec. Additional experiments were per- formed using pure NgO as the absorber and the P(16) and P(24) C02 laser lines in the 10 micron band. These experi- ments show the variation in optoacoustic signal as cell pressure is reduced. A decrease in collisional broadening of the absorbing gas with reduced pressure changes the position of the laser line relative to the absorption profile. This in turn alters the optoacoustic signal as a function of pressure. Conclusions This report establishes a procedure by which the relaxation times correspond- ing to radiative, collisional and diffu- sional processes can be estimated. Applied to optoacoustic detection of trace gases, the procedure shows the potential for optimizing optoacoustic signal response. The means devised for calibrating electret microphone response is appropriate for more general use in accounting for changes in microphone sensitivity. This Project Summary was authored by William McClenny who is also the EPA Project Officer (see below). The complete report, entitled "Effect of Collisional Lifetime in Optoacoustic Detection of Pollutant Gases." was authored by Wolfgang Christian, who was formerly with Allegheny College, Meadville, PA 16335 and is now with Earlham College, Richmond, IN 47374. The above report f Order No. PB 81-173 312; Cost: $6.50, subject to change) wilt be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Environmental Sciences Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Postage and Fees Paid Environmental Protection Agency EPA 335 Official Business Penalty for Private Use $300 1 PS 0000329 US ENVIR PROTECTION AGENCX KEGION 5 LIBRARY 230 $ DEARBORN STREET CHICAGO IL 60604 ------- |