The importance of interstitial gas expansion in infrared photoacoustic spectroscopy of powders

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Abstract
Model calculations and experiments have been performed to explore the modes of photoacoustic signal generation from highly porous powders. These indicate that two modes of signal generation are important: (i) a thermal mode resulting from heating of the sample and conduction to the transducer gas above the sample, and (ii) a pressure mode resulting from the volumetric expansion of the interstitial gas in the porous sample. For highly porous materials, such as silica, the pressure mode can dominate the photoacoustic signal. Under conditions where the pressure mode is dominant, the frequency dependence of the acoustic magnitude and phase is similar to that of a homogeneous sample in photoacoustic saturation, however, the photoacoustic signal will still scale with absorption coefficient in this regime. The pressure mode of signal generation can be attenuated by compaction of the powdered sample to reduce the porosity. Unambiguous determination of photoacoustic saturation is shown to require measurements as functions of both frequency and porosity. The experimental results show that for highly porous silica even the strongest absorption bands are not in photoacoustic saturation. The results also suggest that infrared light scattering by powders of submicron-sized particles is not of major significance. This suggests that photoacoustic spectroscopy is extremely well suited to the study of highly porous materials.