Observation of Large Photoacoustic Signal Phase Changes During a Diffusion Process

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Abstract:

The phase of the photoacoustic signal is known to be a sensitive and accurate means to investigate, both qualitatively and quantitatively, static multilayer heterogeneous systems. According to theory, the maximum phase delay for a very weakly absorbing homogeneous sample should be within 45° of a very strongly absorbing sample, while for heterogeneous samples the phase delay can be greater than 45°. Here we report the observation of photoacoustic phase delays greater than 350° by extending the use of step-scan phase modulation photoacoustic spectroscopy to study a non-repetitive dynamic system in situ, in real time. These large phase delays correspond to sampling several thermal diffusion lengths into the sample. The model system used in this study consisted of a hydrocarbon grease diffusing through a porous Teflon film. The progress of the diffusion was tracked by monitoring both the photoacoustic signal magnitude and the phase of the hydrocarbon grease after isolation from the Teflon film signal contributions at two different phase modulation frequencies.

Keywords: DEPTH PROFILING; DIFFUSION MONITORING; PHASE-RESOLVED PROFILING BY PAS; PHOTOACOUSTIC SPECTROSCOPY

Document Type: Research Article

DOI: http://dx.doi.org/10.1366/000370205774783142

Affiliations: 1: Ames Laboratory, Iowa State University, Ames, Iowa 50011 2: MTEC Photoacoustics, Inc., P.O. Box 1095, Ames, Iowa 50014; Ames Laboratory, Iowa State University, Ames, Iowa 50011

Publication date: November 1, 2005

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