Infrared radiation absorbent absorption spectrum

What happens to a molecule once it has absorbed infrared radiation The absorption causes an increase in the total energy of the molecule, thereby increasing the amplitude of the particular molecular vibration that was responsible for absorption of the radiation. However, no irreversible change occurs in the molecule, as the excess energy is quickly dissipated in the form of heat. Obtaining an IR spectrum thus does not p>ermanently change the molecular structure. 

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

Each dip in the spectrum is called a band or peak. The transmittance is 0 % if all the radiation is absorbed and with no absorption the transmittance is 100%. The absorption of infrared radiations can be expressed either in terms of wave length (A.) or in wave number (y). Mostly infra red spectra of organic compounds are plotted as percentage transmittance against wave number. The relationship between wave length and wave number is as follows ... 

Infrared radiation of frequencies less than about 100 cm-1 is absorbed and converted by an organic molecule into energy of molecular rotation. This absorption is quantized thus a molecular rotation spectrum consists of discrete lines. 

In emission spectrometry, the sample is the infrared source. Materials emit infrared radiation by virtue of their temperature. KirchhofF s law states that the amounts of infrared radiation emitted and absorbed by a body in thermal equilibrium must be equal at each wavelength. A blackbody, which is a body having infinite absorptivity, must therefore produce a smooth emission spectrum that has the maximum possible emission intensity of any body at the same temperature. The emissivity, 8, of a sample is the ratio of its emission to that of a blackbody at the same temperature. Infrared-opaque bodies have the same emissivity at all wavelengths so they emit smooth, blackbody-like spectra. On the other hand, any sample dilute or thin enough for transmission spectrometry produces a structured emission spectrum that is analogous to its transmission spectrum because the emissivity is proportional to the absorptivity at each wavelength. The emissivity is calculated from the sample emission spectrum, E, by the relation... 

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