Terahertz far-infrared spectrometers

Other groups have built tunable far-inffared spectrometers which do not involve high-frequency backward-wave oscillators. Verhoeve, Zwart, Versluis, Drabbels, ter Meulen, Meerts, Dymanus and McLay [61] have described a system in which fixed frequency far-inffared radiation is mixed with tunable microwave radiation in Schottky barrier diodes. This instrument has been operated up to 2.7 THz, and used to study OD and N2H+. A similar system, combined with a continuous supersonic jet, has been described by Cohen, Busarow, Laughlin, Blake, Havenith, Lee and Saykally [62], This instrument was used to study rare gas/water clusters. 

Amano succeeded in observing the lowest rotational transition of the CH radical, generated by means of a glow discharge in mixture of CH4 and helium. The observation of this spectrum is a major achievement in microwave spectroscopy we show a part of the spectrum in figure 10.31, exhibiting two proton hyperfine lines, but defer a detailed discussion of the energy levels and spectrum until later in this chapter. 

It passes through the sample absorption cell made of Pyrex with polyethylene windows and is detected with a liquid helium cooled bolometer. One of the lasers is frequency-modulated at 1 kHz and the detector output is processed with a lock-in amplifier, as shown. Far-infrared rotational spectra of CO, HC1 and HF have been recorded [67], and as an example of the excellent sensitivity achieved, we refer the reader to the spectrum of the OH radical [68] shown later in this chapter. Evenson s spectrometer operates over a wide range of the far-infrared region up to 9 THz, with excellent frequency stability. 

It will be clear from this subsection that much skillful and imaginative instrument design, by a number of different groups, has been directed towards the development of far-infrared spectroscopy. Quite apart from the developments in laboratory spectroscopy, the impact on astronomy in this region of the spectrum is of major importance. A high power tunable far-infrared source can serve as the local oscillator for the detection of far-infrared interstellar radiation. We can anticipate exciting developments in this field. 

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