Far-infrared sources

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. 

It should not be thought that OH always exhibits the unusual behaviour described above the recent developments in tunable far-infrared sources have had an important impact in astronomy, so that interstellar rotational transitions can now be observed. We described an airborne far-infrared telescope in the first part of this chapter, and figure 10.60 shows two examples of interstellar OH rotational transitions, observed by Watson, Genzel, Townes and Storey [170],... 

Commenting on Dreizler s paper concerning determination of barriers to internal rotation, lide emphasized that a combination of data from both microwave and far-infrared sources often gives a better value of the barrier height than either method alone. The quantity determined most accurately from the measurement of splittings in the microwave spectrum is s which is roughly proportional to the product of V and Ia ... 

Synchrotron infrared beamlines have been used for studying a number of exciting condensed matter systems from high temperature superconductors, to VO2, to semiconductors, to graphene. Here we overview some recent work about the fascinating optical properties of graphene and the use of coherent synchrotron radiation as a novel and powerful far-infrared source. 

Lasers are sources of highly collimated, coherent, and intense beams of light that may be obtained commercially from the ultraviolet into the far infrared. 

As for the far-infrared, absorption by air in the vacuum-ultraviolet (VUV) necessitates evacuation of the optical path from source to detector. In this region it is oxygen which absorbs, being opaque below 185 nm. 

The use of vibrational Raman spectroscopy in qualitative analysis has increased greatly since the introduction of lasers, which have replaced mercury arcs as monochromatic sources. Although a laser Raman spectrometer is more expensive than a typical infrared spectrometer used for qualitative analysis, it does have the advantage that low- and high-wavenumber vibrations can be observed with equal ease whereas in the infrared a different, far-infrared, spectrometer may be required for observations below about 400 cm. ... 

Terahertz, or far infrared spectroscopy, covers the frequency range from 0.1 to lOTHz (300 to 3cm ) where torsional modes and lattice vibrations of molecules are detected. It is increasing in use in many application areas, including analysis of crystalline materials. Several dedicated conunercial instruments are available which use pulsed terahertz radiation which results in better signal to noise than those using blackbody sources for radiation (and associated with the terminology far infrared spectroscopy). Work using extended optics of FTIR instrumentation as weU as continuous-wave source THz has also been recently reported. ... 

The far infrared region. The standard infrared (IR) light source used to be the glowbar, but in recent years it was realized that synchroton radiation... 

Fourier transform methods have revolutionized many fields in physics and chemistry, and applications of the technique are to be found in such diverse areas as radio astronomy [52], nuclear magnetic resonance spectroscopy [53], mass spectroscopy [54], and optical absorption/emission spectroscopy from the far-infrared to the ultraviolet [55-57]. These applications are reviewed in several excellent sources [1, 54,58], and this section simply aims to describe the fundamental principles of FTIR spectroscopy. A more theoretical development of Fourier transform techniques is given in several texts [59-61], and the interested reader is referred to these for details. 

The application of classical Raman spectroscopy, using the mercury radiation at A 253.6 nm as the excitation source, permitted recording20 of more than 20 peaks for sucrose below 500 cm-1. The observed frequencies below 100 cm"1 were interpreted as due to inter-ring oscillations, which was also the conclusion reached from a far-infrared study84 of glucose and sucrose. 

MICROWAVE SPECTROSCOPY. A type of adsorption spectroscopy used in instrumental chemical analysis that involves use of that portion of the electromagnetic spectrum hav ing wavelengths in the range between the far infrared and the radiofrequencies, i.e.. between 1 nun and. 111 cm. Substances to be analyzed are usually in the gaseous state. Klystron tubes are used as microwave source. 

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