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
------- |