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. 

Sources of radiation are all of lower than ideal intensity. One of the most commonly used is a mercury discharge in a quartz envelope, most of the higher-wavenumber radiation coming from the quartz rather than from the discharge plasma. 

The windows of the absorption cell are made from polymer material such as polyethylene, poly(ethylene terephthalate Terylene ) or polystyrene. 

A commonly used detector is a Golay cell, in which there is a far-infrared absorbing material, such as aluminium deposited on collodion, inside the entrance window of the cell. 

The aluminium absorbs the radiation, heats up and transfers the heat to xenon gas contained in the cell. As the temperature of the gas varies the curvature of a flexible mirror of antimony-coated collodion, forming a part of the cell, changes. Reflection of a light beam from this mirror, which is on the outside of the Golay cell, indicates its curvature and therefore the intensity of radiation absorbed by the cell. 

The same detector material (Ge Ga) used in midinfrared arrays can also be utilized for small arrays in the far infrared region. It was discovered that if a mechanical stress is placed on the detector material, it increased its wave- 

Both microwave and millimetre wave radiation can be channelled in any direction by a waveguide made from metal tubing of rectangular cross-section, the dimensions depending on the frequency range. The absorption cell is also made from waveguide tubing. 

---

Maranowski K D, Gossard A C, Unterrainer K and Gornik E 1996 Far-infrared emission from parabolioally graded quantum wells Appl. Rhys. Lett. 69 3522-4... 

Varberg T D and Evenson K M 1992 Accurate far-infrared rotational frequencies of carbon monoxide Astrophys. J. 385 763-5... 

Cheville R A and Grioschkowsky D 1999 Far-infrared foreign and self-broadened rotational linewidths of high temperature water vapor J. Opt. See. Am. B 16 317-22... 

Venables D S and Schmuttenmaer C A 1998 Far-infrared spectra and associated dynamics in acetonitrile-water mixtures measured with femtosecond THz pulse spectroscopy J. Ohem. Rhys. 108 4935-44... 

EIOs), backward wave oscillators (BWOs) or magnetrons are available. Their spectral characteristics may be favourable however, they typically require highly stabilized high-voltage power supplies. Still higher frequencies may be obtained using far-infrared gas lasers pumped for example by a CO- laser [49]. 

It is tlius of great interest to carry out experiments tliat excite tire intennolecular bending and stretching vibrations directly, witliout exciting tire monomers as well. These transitions he deep in tire far infrared, typically in tire 20-... 

Tunable visible and ultraviolet lasers were available well before tunable infrared and far-infrared lasers. There are many complexes that contain monomers with visible and near-UV spectra. The earliest experiments to give detailed dynamical infonnation on complexes were in fact those of Smalley et al [22], who observed laser-induced fluorescence (LIF) spectra of He-l2 complexes. They excited the complex in the I2 B <—A band, and were able to produce excited-state complexes containing 5-state I2 in a wide range of vibrational states. From line w idths and dispersed fluorescence spectra, they were able to study the rates and pathways of dissociation. Such work was subsequently extended to many other systems, including the rare gas-Cl2 systems, and has given quite detailed infonnation on potential energy surfaces [231. 

Far-infrared and mid-infrared spectroscopy usually provide the most detailed picture of the vibration-rotation energy levels in the ground electronic state. However, they are not always possible and other spectroscopic methods are also important. 

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... 

Marshaii M D, Charo A, Leung H O and Kiemperer W 1985 Characterization of the iowest-iying n bending state of Ar-HCi by far infrared iaser-Stark spectroscopy and moieouiar beam eiectric resonance J. Chem. Phys. 83 4924-33... 

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

Far-infrared Mercury arc Polymer Grating interferometer Golay cell thermocouple bolometer pyroelectric... 

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 5.3 Far-infrared spectrum of CO showing transitions with J" = 3 to 9. (Reproduced, with permission, from Fleming, J. W. and Chamberlain, J., Infrared Phys., 14, 277, 1974. Copyright 1974 Pergamon Press)...

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)...

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. ... 

Because of fhe presence of heavy atoms in many inorganic molecules fhere may be several low-wavenumber vibrations. For tins reason if is generally more importanf fhan for organic molecules fo obfain fhe far-infrared or Raman spectrum. 

It is important to realize that electronic spectroscopy provides the fifth method, for heteronuclear diatomic molecules, of obtaining the intemuclear distance in the ground electronic state. The other four arise through the techniques of rotational spectroscopy (microwave, millimetre wave or far-infrared, and Raman) and vibration-rotation spectroscopy (infrared and Raman). In homonuclear diatomics, only the Raman techniques may be used. However, if the molecule is short-lived, as is the case, for example, with CuH and C2, electronic spectroscopy, because of its high sensitivity, is often the only means of determining the ground state intemuclear distance. 

---