When an intens beam of light is passed through a liquid or gas, some is scattred by the molecules of the medium. Most of the scattered light has the same frequency as the incident light.
This type of scattering which occurs without a charge in the incident frequency is termed Rayleigh scattering. In 1928, Indian physicist Sir C.V. Raman discovered that a very small frequency of the scattered light has frequency higher or lower than the incident frequency. The scattering which occurs with a change in the frequency of the incident frequency. The scattering which occurs with a change in the frequency of the incident radiations is called Raman scattering.The phenomenon is called Raman effect.
The spectrum obtained from scattered radiations of a sample can be observed with the help of the apparatus. Laser beam is now used ad incident radiation. Raman originally used ultraviolet light which gave weak spectral lines.
The general pattern of spectrum recorded by scattered light is illustrated. The sharp line in the middle corresponds to Rayleigh scattered light and is referred to as Rayleigh line. Raman scattered light record a series of lines on either side of the Rayleigh line.
These are called Raman lines and constitute Raman spectrum.
Explanation of Raman Spectrum
The change in the frequency by scattering occurs due to the exchange of energy between the incident photon and the scattering molecules. If the photon collides with an 'excited' molecule, it acquires energy from the molecule. Thus the scattered light emerges a higher frequency. If the photon collides with a molecule in the 'ground state', if loses energy to molecule. This lowers the frequency of the scattered light. It explains why Raman spectrum of a substance shown lines of higher frequency and other of lower frequency than the incident line. The frequency difference between each Raman line and the incident line, is called the Raman frequency.
The Raman frequencies shown by the substances are indipendent of the incident frequency and are characteristic of the substances.
Raman Spectra and Molecular Structure
Raman spectra provide information about the shape and structure of molecules.
When studies together, Raman and IR sepectra can tell us whether:
(1). A molecules is linear or non-linear.
(2). A molecule is symmetrical or asymmetrical.
The most important application of Raman spectra is in the determination of structure of organic molecules. Defined frequencies are associated with different types of bond.
