Instrumentation for Far‐Infrared Spectroscopy

Abstract The various components of far‐infrared spectrometers are discussed in this article. For measurements above 100 cm −1 , a standard silicon carbide (Globar) source may be used. For longer wavelengths, a high‐pressure mercury lamp is required. When higher radiance is required, synchrotrons emit much higher far‐infrared photon fluxes than either the Globar or the mercury lamp. Pneumatic (Golay) detectors were popular as far‐infrared detectors, as long as the modulation frequency was low. Despite their relatively low sensitivity, pyroelectric bolometers such as the deuterated triglycine sulfate (DTGS) detector are more commonly used because of their better high‐frequency response, which is necessary for Fourier transform infrared (FT‐IR) spectrometry. When very high sensitivity is required, liquid‐helium cooled bolometers are used. Most far‐infrared spectra are measured using an interferometer. In the early days of FT‐IR spectrometry, most interferometers were of the slow continuous‐scan or simple step‐scan variety; lamellar grating interferometers were used for very long‐wavelength measurements. Today, most far‐infrared FT‐IR spectrometers incorporate rapid‐scan interferometers of the same type used for mid‐infrared measurements. A number of special‐purpose instruments are also described, including the Genzel interferometer. All these instruments must be equipped with beamsplitters that are configured for far‐infrared operation. Poly(ethylene terephthalate), also known as Mylar or Melinex, is the most common material used to fabricate far‐infrared beamsplitters but other materials have better efficiency. These include solid silicon or silicon that has been vapor‐deposited onto Mylar.