Mid-infrared radiation

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

A fiber-optic device has been described that can monitor chlorinated hydrocarbons in water (Gobel et al. 1994). The sensor is based on the diffusion of chlorinated hydrocarbons into a polymeric layer surrounding a silver halide optical fiber through which is passed broad-band mid-infrared radiation. The chlorinated compounds concentrated in the polymer absorb some of the radiation that escapes the liber (evanescent wave) this technique is a variant of attenuated total reflection (ATR) spectroscopy. A LOD for chloroform was stated to be 5 mg/L (5 ppm). This sensor does not have a high degree of selectivity for chloroform over other chlorinated aliphatic hydrocarbons, but appears to be useful for continuous monitoring purposes. 

Near-infrared spectroscopy (NIR) works in the 800 nm-2.5 pm (12,500— 4,000 cm-1) range. The advantage of NIR is that it can typically penetrate much farther into a sample than the mid-infrared radiation (30-1.4 pm, 4,000-400 cm-1)- It can be used for the quantitative measurement of organic functional groups of soil organic matter, especially O—H, N—H, and C=0 (Siesler et al. 2002). In addition, the structural modifications under the effect of chemical treatments (e.g., acidic treatments) can also be studied by NIR (Madejova et al. 2009). 

By far the most common use of mid-infrared radiation for process analysis is in the non-dispersive infrared analysers that are discussed below. The widespread use of FTIR spectrometers in the mid-lR has yet to be fully realized in process analytical apphcations. The requirements for the optical components and the wavelength sta-bihty of the instraments available have, until recently, detracted from the use of this region of the spectrum in on-line process analysis. Optical fibers that provide such a benefit to the apphcations of NIR (see below) are not available for the mid-IR in robust forms or forms that are capable of transmitting over more than a few tens of metres. Improvements and developments to sample cells, particularly designs of attenuated total reflectance (ATR) cells, for use with mid-lR are being made and will influence the application of the technique. An impressive list of apphcations including both FTIR and the NDIR approaches has been compiled (2, 3]. 

Source. This usually consists of a filament or rod of some refractory material, heated to a temperature of around 1500 K so as to emit infrared radiation. The Globar is probably the most common source of mid-infrared radiation, consisting of synthetic silicon carbide. This usually has to be water-cooled, however. Filament (Nernst), and nichrome wires are also popular—and may not require water cooling. Water-cooled sources should deliver a higher and more stable output, which is better suited to quantitative applications. 

Finally, as shown in Figure 6-24a, the less ener-getic near- and mid-infrared radiation can bring about transitions only among the k vibrational levels of the ground state. Here, k potential absorption frequencies are given by k equations, which may be formulated as... 

Figure 2.9 Plot demonstrating the small spot size that can be achieved using synchrotron-sourced mid-infrared radiation. The plot represents the integrated signal intensity from 2000-9000 cm through a 10 pm pinhole scanned on a microscope stage in an FT-IR spectrometer. Reproduced from reference [9] by kind permission of the Advanced Light Source (ALS), Berkeley Laboratory.

The limitations Introduced by the minute aperture have meant that few papers have been published on the topic of aperture-based SNOM systems. However, Schnell et al. [3] recently reported how mid-infrared radiation could be focused Into a very small area through a tapered transmission line. Nanofocusing of 10.7 pm radiation was achieved by propagating a mid-infrared surface wave along a tapered two-wire transmission line. The spot diameter was compressed to 60 nm (A/150) at the taper apex. To the best of our knowledge, no nanoimaging results in which this approach has been applied have yet been reported. Most of the major advances in this field have been based on either photothermal (PT) spectroscopy or elastic scattering from a tip. In the remainder of this section, the early work that laid the foundations for some of the remarkable results that have been reported in the past decade will be described. 

Vibrational microspectrometry will undoubtedly be applied to medical diagnosis in the near future. One particularly important application of microspectrometry is for the characterization of tissue samples. Tissue samples can be mounted on a water-insoluble infrared-transparent window such as ZnSe, but these windows are expensive and not conducive to visual examination (e.g., after staining of the tissue). A convenient alternative to transmission spectrometry is the measurement of the transflectance spectrum (see Section 13.5) of tissue samples mounted on low-emissivity glass slides [4]. These slides are transparent to visible light but highly reflective to mid-infrared radiation. 

An infrared spectrum may be defined as a sample-dependent change induced on the intensity distribution of infrared radiation emitted by a source over the entire infrared region. The intensity distribution (spectrum) of the mid-infrared radiation emitted by a source is shown in Figure 3.2a. This spectrum does not have absorptions by any sample (except for absorptions of atmospheric water vapor and carbon dioxide), and it is denoted as the reference spectrum (sometimes called the single-beam background spectrum) B v). ( ) is deter-... 

The source of radiation A Globar, which is usually used as the source for the mid-infrared radiation, can still be used in the wavenumber region of 400-150 cm . However, in order to measure the wavenumber region lower than 150 cm , the Globar needs to be replaced by a high-pressure mercury lamp (see Section 5.2.1.2). 

Infrared microscopes are used to take infrared spectra of small samples and are typically quality visible-light microscopes that have been re-engineered to work with mid-infrared radiation. In many cases the infrared microscope mounts on the side of... 

Near Infrared A type of electromagnetic radiation that is higher in energy than mid-infrared radiation and falls from 14,000 to 4000 cm . 

An alternative to the spherically symmetric optically thin shell of large grains considered above is an anisotropic distribution of optically thick dust condensations. Anisotropy has exciting implications for relating dust distributions to general questions of outflows and of planetary formation in young steUax objects. However, optically thick disks of the kind usually considered in standard disk models are expected to radiate more mid-infrared radiation than we observe from the core component of LkHa 101. 

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