Instrumentation for Far-infrared Spectroscopy

Measurements of far-infrared spectra are made far more difficult than the corresponding measurement of midor near-infrared spectra because of the weakness of the source. The spectral energy density of a blackbody at 1400 K is almost 5800 times weaker at 10 cm 1, and about 60 times weaker at 100 cm 1, than it is at 1000 cm 1. Since the sensitivity of any measurement of a midor far-infrared spectrum is directly proportional to the spectral energy density of the source, it is apparent that far-infrared sources should be as hot as possible and have an emittance of close to unity. For measurements between about 400 and 100 cm 1, the radiant power emitted by a silicon carbide (Globar) source is as high as any conventional infrared source. Although Nernst glowers can be operated at higher temperature than Globars, they become quite transparent below about 200 cm 1, so that their emissivity drops to the point that they are of little use as far-infrared sources, despite their high temperature. Below 100 cm 1, the emissivity of a Globar also becomes low and it is customary to use a high-pressure mercury lamp for measurements between 3⁄450 cm 1 and the onset of the microwave region of the spectrum. The reason why mercury lamps have proved to be so successful for far-infrared spectrometry is because emission from the plasma reinforces the emission from the hot quartz envelope of the lamp. However, the mercury lamp has significant flicker noise, so even if the energy is better, the signal-to-noise ratio (S/N) may not be. It should also be noted that microwave sources have been developed that emit highly monochromatic radiation at shorter wavelengths