Peak wavelength, infrared radiation

As with electronic spectra, the use of infrared spectra for quantitative determinations depends upon the measurement of the intensity of either the transmission or absorption of the infrared radiation at a specific wavelength, usually the maximum of a strong, sharp, narrow, well-resolved absorption band. Most organic compounds will possess several peaks in their spectra which satisfy these criteria and which can be used so long as there is no substantial overlap with the absorption peaks from other substances in the sample matrix. 

From this equation it can be seen that the depth of penetration depends on the angle of incidence of the infrared radiation, the refractive indices of the ATR element and the sample, and the wavelength of the radiation. As a consequence of lower penetration at higher wavenumber (shorter wavelength), bands are relatively weaker compared to a transmission spectrum, but surface specificity is higher. It has to be kept in mind that the refractive index of a medium may change in the vicinity of an absorption band. This is especially the case for strong bands for which this variation (anomalous dispersion) can distort the band shape and shift the peak maxima, but mathematical models can be applied that correct for this effect, and these are made available as software commands by some instrument manufacturers. 

In a typical spectrophotometer, a dissolved compound is exposed to electromagnetic radiation with a continuous spread in wavelength. The radiation passing through or absorbed is recorded on a chart against the wavelength or wave number. Absorption peaks are plotted as minima in infrared, and usually as maxima in ultraviolet sp>ectroscopy. 

The peak power of infrared radiation at 300 K has a wavelength of 10 microns. This presents the problem that an infrared radiation detector operating in this range will quickly be saturated because many surrounding objects will be at 300 K. The invention of US-A-4807007 refers to metal-insulator-semiconductor, MIS, detectors, each having a potential well of which only a small portion is exposed to infrared radiation. 

Infrared pyrometer -pI- ra-mo-tor n. A narrow- or broad-band instrument that senses the peak wavelength, A (pm) of IR radiation emanating from a warm surface. By Wien s displacement law, the absolute temperature (K) is given by 2884/2. 

Infrared spectra are obtained by passing infrared radiation through the sample of interest and observing the wavelength of absorption peaks. These peaks are caused by the absorption of the electromagnetic radiation and its conversion into specific molecular motions, such as C—stretching. 

Figure 10.1 Infrared radiation sources on a jet aircraft and their peak emission wavelength together, and the polar diagram of radiant intensity at various aspect angles.

All astronomical objects emit infrared radiation. In fact, the peak of the integrated light from all of the stars in a normal galaxy is near a wavelength of 1 /xm. The amount of infrared radiation emitted by a star, planet, or other astronomical object is determined by its temperature, the blackbody or Planck curve, and its emissivity at each wavelength, as given in Eq. (1.2). 

A red-hot object has a spectrum that peaks around 675 nm, whereas a white-hot object has a spectrum that has comparable intensities for all wavelengths in the visible region. The sun has a blackbody temperature of about 5750 K. Objects emit radiation at a//temperatures, not just at high temperatures. For example, night-vision goggles make infrared radiation emitted by objects visible in the dark. 

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