Research-quality far-infrared photoacoustic (PA) spectra are obtainable with a Fourier transform infrared spectrometer, the only changes with respect to conventional mid-infrared PA spectroscopy being the use of (1) a caesium iodide or polyethylene window on the PA cell, and (2) a mylar beamsplitter. Far-infrared PA spectra of several solids (bentonite, Fe+3-bentonite, and asbestos), in addition to the PA reference carbon black, have been recorded in this way. In order to improve signal-to-noise ratios in one of the spectra, we recorded ten interferograms under identical conditions; it was found that the average of the ten individually calculated spectra displays less noise and fewer spurious features than the spectrum obtained by first averaging the interferograms and then calculating a single spectrum. The results of this investigation demonstrate the feasibility of far-infrared PA spectroscopy, and illustrate that both experimental and computational procedures should be optimized in order to obtain the most satisfactory spectra.
J. C. Donini and K. H. Michaelian, "Low-Frequency Photoacoustic Spectroscopy of Solids," Appl. Spectrosc. 42, 289-292 (1988)