When chopped light impinges on a solid in an enclosed cell, an acoustic signal is produced within the cell. This effect is the basis of a new spectroscopic technique for the study of solid and semisolid matter. A quantitative derivation is presented for the acoustic signal in a photoacoustic cell in terms of the optical, thermal, and geometric parameters of the system. The theory predicts the dependence of the signal on the absorption coefficient of the solid, thereby giving a theoretical foundation for the technique of photoacoustic spectroscopy. In particular, the theory accounts for the experimental observation that with this technique optical absorption spectra can be obtained for materials that are optically opaque.
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A recent treatment of the photoacoustic effect with solid by
Parker
[Appl. Opt.
12
, 2974
(1973
)] also assumes that the signal is produced by heat conduction from solid sample to surrounding gas. His treatment, however, assumes an anomalously high absorption coefficient for a thin surface layer of the solid.14.
A more exact treatment for the gas can be derived using equations of fluid dynamics. However, the approximate treatment given here gives results consistent with the more exact method, and has the advantage of presenting a simpler physical description of the process.
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It should be noted that we have not considered here the effects of reflection losses at the surface.
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© 1976 American Institute of Physics.
1976
American Institute of Physics
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