Normalizing Infrared Photoacoustic Spectra of Solids

Infrared spectra of infrared detectors were measured in order to contribute to our understanding of the normalization of photoacoustic spectra of solids1. Such spectra are usually “source-compensated,” i.e., normalized to account for differences in source emission at different wavelengths, and such compensation or normalizing is done by comparing the single-beam spectrum of the sample to the emission spectrum of the source measured with a detector or, more commonly, to the single-beam spectrum of a carbon material of some sort which is taken to be a flat black absorber. We showed, however, that this assumption was not generally valid because different carbons gave different spectra and, because of the variability of carbons, concluded that it would be better to avoid carbons and use direct measurements of the exciting source to compensate spectra.2 That statement was not further amplified partly because we did not know how absolute direct measurements might be made. These conclusions were supported by Riseman and Eyring3, who measured IR Fourier transform photoacoustic spectra of various carbons and found not only that the spectra differed but that each individual carbon would give rise to varying spectral responses depending on the scan speed of their interferometer. In also concluding that no form of carbon was suitable for source normalization they suggested that “…a more easily standardized normalization method for FTIRPAS would employ a double beam in space arrangement where a fraction of the incoming beam is directed in to th e DTGS [deuterated triglycine sulfate] detector, or any detector whose response function closely matches that of an ideal photoacoustic black body absorber, ”3 i.e., the DTGS or some other detector is used to measure the source spectrum directly.