Photoacoustic spectroscopy of thick powdered or porous samples at low frequency

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Abstract
This paper sets forth a self-contained theoretical description of the photoacoustic effect in a thick powdered or porous sample at low modulation frequencies where the sample medium can be considered homogeneous from the point of view of heat diffusion. It is shown that the photoacoustic signal results from two contributions: the heat flux from the sample to the gas plus the expansion of the interstitial gas and that the pressure is essentially uniform throughout the sample. Scattering effects are introduced with a Kubelka–Munk modified model and an expression is first given for the conventional partially filled cell. We discuss of these results and their application to the explanation of several texture effects and to the analysis of experimental data. Using a sample composed of chrysotile asbestos powder, we present experimental data and perform an approximate analysis to retrieve the absorption spectrum. Several experimental results are also presented and thoroughly discussed for a cell completely filled with powder. The relationships between photoacoustic spectra and diffuse reflectance spectra are given for both cases.