Applications in photoacoustic spectroscopy




Ligday, Robert Carl

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This work demonstrates the range of applications for the photoacoustic effect in absorption spectroscopy. The advantage of studying optical absorption by measuring the thermal energy produced from an electronic relaxation without scattered or reflected light interferences permitted the investigations of solid opaque materials and thin films on opaque electrodes. Previously absorption studies of this sort were carried out using transmission spectroscopy of very dilute solutions, or with thin films on transparent substrates. The complication involved in the PA techniques stem from the intertwining of absorption and thermal effects. Difficulties of this sort were observed in the saturation of the PA signal with the highly colored dyes rose bengal and p-nitroaniline, and in the PA spectra of the series of reduced TiO₂ powders and crystals. PA techniques, coupled with electrochemical systems, produced both absorption spectra and a possible means of determining thermal and optical parameters of the materials involved. The fast coloration/ bleaching process of the WO₃/H[subscript x]WO₃ system compared to the slower, thermally dependent PA responses could provide thermal diffusion data for both thin films and their backing electrodes by measurement of the time delay of the PA signal relative to the time of coloration. It was also shown that from the changes in the PA response to different thickness of films (WO₃), absorption coefficients of that film could be determined. Techniques such as piezoelectric detection [37] of the thermal energy at the back of an electrode generated from photoabsorption of a thin film have produced similar results with much higher signal magnitudes. The TiO₂ characterization study has left several questions unanswered. Although trends show the PA signal to increase upon the reduction of rutile and anatase samples, it is not confirmed that it is entirely due to the optical absorption. The complications of particle sizes and surface area effects and the possibility of changes in the thermal parameters accompanying the reduction have not been resolved. An experiment with a single rutile crystal treated similarly to the treatment of the powders and crystals reported here could eliminate the particle size and surface area effects, and would allow optical transmission spectra to identify changes in absorption


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