Absorption in infrared region of spectrum associated with changes molecular energy. Those vibrations which involve a change in dipole moment produce a periodically changing electromagnetic field, which will interact with electromagnetic radiation of the same frequency so that it is absorbed, and the intensity of absorption will be proportional to the square of rate of change of dipole moment.
Infrared spectroscopy involves the twisting, bending, rotating, and vibrational motions of the atoms in a molecule. Upon the interaction with infrared radiation, portion of the incident radiation are absorbed in particular wavelengths. The multiplicity of vibrations occurring simultaneously produces a highly complex absorption spectrum, which is uniquely characteristic of the functional groups comprising the molecule and the overall configuration of the atoms as well.
A vibrating molecule may interact in two distinctly different ways with electronic radiation of appropriate frequency. If the radiation has the same frequency as one as the normal modes in vibration means that, It will be in the infrared region of electronic spectrum and it possible to absorb the radiation. An infra red absorption spectrum of a material is simply by allowing infrared radiation to pass trough the sample and determining what fraction is absorbed at the each frequency within some particular range. The frequency at which any peak in the absorption spectrum appears is equal to the frequency of one of the normal modes of vibration of the molecules of the sample.
The Fourier Transform Infrared Spectrometer
The Fourier transform infrared (FTIR) spectrometer, which is usually based on the Michelson interferometer, provides a completely different approach by recording of infrared spectra.
The effect of the mirror in monochromatic radiation of the wavelength passes trough in interferometer. When the path difference between the two beams is an odd integral multiply λ/2 destructive interference will be occur for radiation transmitted in the direction of the detector and measured intensity of the recombine beams will be zero. When the difference is nλ, where n is integer, the two beams constructively interfere, to give maximum measured intensity. The intensity measured by detector varies consinusoidally with the displacement of the mirrors. If radiating passing trough the interferometer and reach the detector consist of two wavelength of equal intensity of the combined beam at the detector is a more complex but still symmetrical function of the displacement of the mirror. When two wavelength are of unequal intensity owing the pattern is further modified
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2.Bower, Maddams (1989), The Vibrational Spectroscopy of Polymer, Cambridge University Press.
3.Bellamy, L.J, Advances in Infrared Group Frequencies, Chapman and Hall, London.
4.Browning, D.R, Spectroscopy, McGraw Hill, London.