The Far Infrared

Over the next few years, both the mid-infrared and the far-infrared spectra for Ar-HF and Ar-HCl were extended to numerous other bands and to other isotopic species (most importantly those containing deuterium). In 1992, Hutson [18, 39] combined all the available spectroscopic data to produce definitive potential energy surfaces that included both the angle dependence and the dependence on the HF/HCl monomer vibrational quantum number v... 

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

In the far-infrared region strong absorption by the water vapour normally present in air necessitates either continuously flushing the whole optical line with dry nitrogen or, preferably, evacuation. 

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. 

In practice, what is measured experimentally is not energy but frequency, in the millimetre wave and microwave regions, or wavenumber, in the far infrared. Therefore we convert the energy levels of Equation (5.10) to what are known as term values F J) having dimensions of either frequency, by dividing by h, or wavenumber, by dividing by he, giving... 

Figure 5.10 Part of the far-infrared spectrum of silane. (Reproduced, with permission, from Rosenberg, A. and Ozier, I., Can. J. Phys., 52, 575, 1974)...

In the near-infrared, Al Ga As and In Ga As P lasers, and in the far-infrared lead compound semiconductor lasers are tunable by varying temperature and operating current. Many exceUent spectroscopic studies have been performed using them. However, they do have relatively limited tuning ranges for any one device. 

We have determined the optical properties as a function of temperature by measuring the reflectivity / (co) of the oriented CNT films from the far infrared (FIR) up to the ultra-violet (UV) (i.e., from 20 cm" up to 3x10 cm" ), using three spectrometers with overlapping frequency ranges [12, 13],... 

In the case where x and y are the same, C (r) is called an autocorrelation function, if they are different, it is called a cross-correlation function. For an autocorrelation function, the initial value at t = to is 1, and it approaches 0 as t oo. How fast it approaches 0 is measured by the relaxation time. The Fourier transforms of such correlation functions are often related to experimentally observed spectra, the far infrared spectrum of a solvent, for example, is the Foiuier transform of the dipole autocorrelation function. ... 

The thermal decomposition reactions of KN3, T1N3, and AgN3 have been studied in the corresponding halide matrices [301]. The formation of NCCT from trapped C02 was described and labelling with ISN established that only a single end-N atom of the azide ion was involved in NCO formation. The photodecomposition of PbN6 and the effects of dopants have been followed [302] by the changes produced in the near and the far infrared. 

Guillot B., Bratos S. Theoretical analysis of dielectric properties of polar liquids in the far-infrared spectral range, Phys. Rev. A 16, 424-30 (1977). 

Lightman A., Ben-Reuven A. Line mixing by collisions in the far-infrared spectrum of ammonia, J. Chem. Phys. 50, 351-3 (1969) Cross relaxation in the rotational inversion doublets of ammonia in the far infrared, J. Quant. Spectrosc. Radiat. Transfer 12, 449-54 (1972). 

Conversely, in the summer, it would still have a high transparency for the visible, but a high reflectivity for the near infrared and a high emissivity for the far infrared. The present state of the art of thin-films deposition still falls short of this goal which may have to wait for the development of suitable photochromic coating materials. 

Since the vibrational spectra of sulfur allotropes are characteristic for their molecular and crystalline structure, vibrational spectroscopy has become a valuable tool in structural studies besides X-ray diffraction techniques. In particular, Raman spectroscopy on sulfur samples at high pressures is much easier to perform than IR spectroscopical studies due to technical demands (e.g., throughput of the IR beam, spectral range in the far-infrared). On the other hand, application of laser radiation for exciting the Raman spectrum may cause photo-induced structural changes. High-pressure phase transitions and structures of elemental sulfur at high pressures were already discussed in [1]. 

Matrix Raman spectroscopy allows detection of some additional vibrations which are inactive in IR spectra (e.g. symmetrical vibrations vi in AB3 molecules having 3 symmetry) or which tie in the far infrared region. In practice, matrix-isolated organic intermediates have not been studied by Raman spectroscopy the main objects of these investigations are inorganic molecules (AICI3, PbS, Gep2, SiO, etc.) which are evaporated from solids in effusion cells. 

Broad optical transparency from the deep ultraviolet to the far infrared... 

Morley S, Emde von der M, Zahn DRT, Offermann V, Ng TL, Maung N, Wright AC, Fan GH, Poole IB, Williams JO (1996) Optical spectroscopy of epitaxial Ga2Se3 layers from the far infrared to the ultraviolet. J Appl Phys 79 3196-3199... 

Due to the unavailability of appropriate cells, the Sn-F absorptions in the far infrared region cannot be observed. 

Examinations of the far-infrared spectra of solutions of Zr (allyl) 3CI and Zr (allyl) 2CI2 (9) suggest that the former exists in solution as the dimer (X), whereas the latter has the monomeric structure (XI). A broad intense peak at 244 cm-1 can be assigned to zirconium-bridging chlorine stretching mode. This band is completely absent from the spectrum of the dihalide and is replaced by a very strong band at 342 cm-1 due to the nonbridging chlorine. 

Hence, in the case of a cryogenic apparatus, e(A) (e = 1 for a black body) must be known in the far infrared and is different from the value in the visible region. However, well-polished metals are usually good reflectors even in the far infrared. 

Figure 5.3 shows a metallic cryostat with LN2 shield used for optical measurements in the far infrared (an optical filter in this case is inserted on the 77K to block the visible radiation). 

This type of 3He refrigerator was developed for astrophysical measurements in the far infrared on board of stratospheric balloons, rockets [23] and aeroplanes and also for ground-based telescopes [24],... 

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