Infrared absorptions/spectra important

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. Photoacoustic detection has been applied to trace-level gas measurements and commercial instruments are available based on this mode of detection. It is important to note that the photoacoustic spectrum is a direct measurement of infrared absorption. While most infrared spectra are either directly or indirectly correlated... 

The infrared spectrum (especially the near-infrared) has assumed great importance in chemical and biological research because of the highly specific absorption of chemical compounds at these wavelengths. The infrared absorption of a given organic compound may be used to characterize that particular compound. The infrared spectrum of a mixture of several compounds among which there is no interaction, does not lie between the spectra of the individual compounds, but consists of a direct superposition of the spectra of the individual compounds... 

For a reasonable set of the parameters the calculated far-infrared absorption frequency dependence presents a two-humped curve. The absorption peaks due to the librators and the rotators are situated at higher and lower frequencies with respect to each other. The absorption dependences obtained rigorously and in the above-mentioned approximations agree reasonably. An important result concerns the low-frequency (Debye) relaxation spectrum. The hat-flat model gives, unlike the protomodel, a reasonable estimation of the Debye relaxation time td. The negative result for xD obtained in the protomodel is explained as follows. The subensemble of the rotators vanishes, if u —> oo. 

Whereas ATR spectroscopy is most commonly applied in obtaining infrared absorption spectra of opaque materials, reflection-absorption infrared spectroscopy (RAIRS) is usually used to obtain the absorption spectrum of a thin layer of material adsorbed on an opaque metal surface. An example would be carbon monoxide adsorbed on copper. The metal surface may be either in the form of a film or, of great importance in the study of catalysts, one of the particular crystal faces of the metal. 

Solvents used in ultraviolet, visible, infrared, microwave, and radiowave spectroscopy must meet the following requirements transparency and stability toward the radiation used, solubility and chemical stability of the substance to be examined, and a high and reproducible purity ( optical constancy ). Normally, intermolecular interaction with the solute should be minimal. On the other hand, important information about the solute can be obtained from the changes in the absorption spectrum arising from such interactions. 

After water vapor and C02, methane (CH4) is the third most important greenhouse gas. Each additional molecule of CH4 added to the atmosphere absorbs about 20 times as much long-wave infrared radiation as does a molecule of carbon dioxide. This occurs in part because some of the absorption spectrum of methane lies in windows in the carbon dioxide absorption spectrum (see Fig. 4-42) therefore, methane absorbs wavelengths that are not already being highly attenuated by carbon dioxide. Currently, the global concentration of methane in the atmosphere is approximately 1.7 ppm and is increasing at an annual rate of approximately 0.01 ppm per year (Table 4-14). The seasonal fluctuations shown in Fig. 4-44 may correspond to seasonal... 

One of the important functions of this infrared microscope is the measurement of the IR spectrum from a spatial region smaller than the diffraction limit. This possibility is already illustrated in Figure 29.4e. The TFD-IR spectrum, that corresponds to the IR absorption spectrum, was measured from a fluorescence region smaller than the IR diffraction limit. Infrared spectroscopy in a sub-micron region will be possible by using a high NA objective lens with the confocal optical system. 

Atmospheric N20 was discovered in 1938 by Adel via infrared absorption features in the solar spectrum. For the next 30 yr, N20 aroused little interest, presumably because it is neither a hazardous pollutant nor does it display any particular chemical activity. In fact, there are no gas-phase reactions that remove it from the troposphere as far as we know. In the stratosphere (see Chapter 3), N20 undergoes photodecomposition, and it reacts with O( D). The second reaction is the major source of higher nitrogen oxides in that region, and since these reduce ozone catalytically via chain reactions, N20 is an important agent in controlling the stratospheric ozone balance. The recognition of this relationship by Crutzen (1970, 1971) and McElroy... 

---