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

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   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
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