Phase Analysis in Solid-Sample Optoacoustic Spectrometry

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Abstract
The optoacoustic effect was discovered by Bell in 18811 and since the early investigations of this phenomena many workers have employed the effect for gas analysis2,3 and the study of gaseous reaction intermediates4. The corresponding applications of optoacoustic spectrometry (OAS) to solid samples however, has received little attention, and it is only in recent years that analytical data for solid samples have been reported. Harshbarger and Robin5, Rosencwaig6 and Adams et al 7 have published the results of studies demonstrating the application of OAS to solid samples and have discussed the advantages of this technique over more conventional optical spectroscopic methods of analysis. In each of the systems described for the examination of solids by OAS, UV/visible radiation from a high power continuum source is modulated using a rotating sector and, after dispersion using a monochromator, is incident upon the sample mounted within a suitable closed cell which also contains a microphone transducer. Energy absorbed from the source may appear as heat in the sample and give rise to a periodic pressure rise in the gas which surrounds the sample. This periodic pressure rise is monitored by the microphone. When the wavelength of the incident radiation is scanned a spectrum of the sample is obtained which is the product of the optical electronic absorption spectrum of the sample and the emitted power spectrum of the source employed. To correct for the variation in incident radiant power with wavelength it is necessary to normalize the spectra obtained by reference to a blackbody absorber. This normalization may be achieved manually7, with a stored pre-recorded reference spectrum6 or simultaneously during the wavelength scan using a double-beam spectrometer.