The Rosencwaig-Gersho equation for the photoacoustic signal is recast in a manner that emphasizes the crucial role thermal wave interference plays in the production of the photoacoustic signal. This formalism is then used to suggest a technique for extracting thermal information from the structure in the photoacoustic signal resulting from thermal wave interference. Experimental measurements illustrating this technique are presented.
A carbon dioxide laser source was used to determine absorption coefficients for dilute absorber-air mixtures at wavelengths corresponding to several vibration-rotation lines in each branch of the 00 degrees 1-02 degrees 0 band at 9.4 microm and the 00 degrees 1-10 degrees 0 band at 10.4 microm. For all samples the total pressure was 1 atm and the temperature was 300 K; the concentrations ranged from 10 ppm (parts per million by volume) to 357 ppm for NH(3) and C(2)H(4), and from 10 ppm to 80 ppm for O(3). The absorption coefficients are tabulated, and the use of selected laser lines in monitoring ambient concentrations is discussed.
The Colorado State University Aerosol Workshop provided an excellent opportunity to obtain various particulate samples collected on filters. Our results indicate that the photoacoustic technique is preferable to the transmission technique (integrating plate method) for ambient samples with low-filter loadings since the presence of a nonabsorbing scattering aerosol (ammonium sulfate) only slightly perturbs the photoacoustic signal and significantly affects the transmitted signal. Measurements indicate that the photoacoustic signal depends not only on the energy absorbed from the incident beam but also on the existence of thermal wave interference effects and, especially for heavily loaded filters, on the presence of a nonabsorbing scattering aerosol.
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