Raman effect

Introduction to Raman effect, Classical and quantum theory

1. Raman Effect

2. Light and Spectroscopy Light interacts with matter in different ways, transmitting through some materials, while reflecting or scattering off others. Both the material a n d t h e c o l o r ( w av e l e n g th) of t h e light affect this interaction. We call the study of this light ‘spectroscopy '. Which parts of the visible spectrum enter our eyes determines which colors we perceive .

3. Raman Spectroscopy Raman spectroscopy is an analytical technique where scattered light is used to measure the vibrational energy modes of a sample. It is named after the Indian physicist C. V. Raman who, together with his research partner K. S. Krishnan, was the first to observe Raman scattering in 1928. Sir Chandrashekhar Venkataraman was known mainly for his work in the field of light scattering for this ground breaking work in the field of light scattering, for which he won 1930 Nobel prize and was also awarded Bharat Ratna.

4. Raman spectroscopy can provide both chemical and structural information, as well as the identification of substances through their characteristic Raman ‘fingerprint’. Raman spectroscopy extracts this information through the detection of Raman scattering from the sample

5. Sir C.V. Raman discovered that when a beam of monochromatic light was allowed to pass through a substance in the solid, liquid or gaseous state, the scattered Light contains some additional frequencies apart from that of incident frequency. This is Known as Raman Effect. The spectrum of scattered light is characteristic property of the scattering substance and is called as Raman spectrum of the molecule

6. Raman Scattering When light is scattered by molecule, the oscillating electromagnetic field of a photon induces a polarization of the molecular electron cloud. This leaves the molecule in a higher energy state with the energy of the photon transferred to the molecule . This can be considered as the formation of a very short-lived complex between the photon and molecule which is commonly called the virtual state of the molecule. The virtual state is not stable and the photon is re-emitted almost immediately, as scattered light.

7. Rayleigh Scattering In the vast majority of scattering events, the energy of the molecule is unchanged after its interaction with the photon. The energy, and therefore the wavelength, of the scattered photon is equal to that of the incident photon. This is called elastic (energy of scattering particle is conserved) or Rayleigh scattering and is the dominant process .

8. Raman Scattering In a much rarer event (approximately 1 in 10 million photons)Raman scattering occurs, which is an inelastic scattering process with a transfer of energy between the molecule and scattered photon. If the molecule gains energy from the photon during the scattering (excited to a higher vibrational level) then the scattered photon loses energy and its wavelength increases which is called Stokes Raman scattering (after G. G. Stokes).

9. Inversely, if the molecule loses energy by relaxing to a lower vibrational level, the scattered photon gains the corresponding energy and its wavelength decreases; which is called Anti-Stokes Raman scattering. Quantum mechanically Stokes and Anti-Stokes are equally likely processes. However, with an ensemble of molecules, the majority of molecules will be in the ground vibrational level (Boltzmann distribution) and Stokes scatter is the statistically more probable process .

10. As a result, the Stokes Raman scatter is always more intense than the anti-Stokes and for this reason, it is nearly always the Stokes Raman scatter that is measured in Raman spectroscopy. The wavelength of the Raman scattered light will depend on the wavelength of the excitation light. This makes the Raman scatter wavelength an impractical number for comparison between spectra measured using different lasers .

11. Characteristics of Raman Effect The frequencies of Raman lines depend on the frequency of incident light. The displacements of Raman lines from the original line depend on the nature of scattering substance. The displacements of Raman lines from the original line is independent of frequency of incident light. The Anti stokes lines are weaker than stokes lines.

12. The vibrational, rotational or electronic energy of a molecule is changed due to scattering of light by it. Raman effect is purely a molecular phenomenon Raman lines are strongly polarized. Raman line are characteristics of scattering material. Raman line are symmetrically displaced about parent lines. Raman shift lies within the far infrared and near infrared regions

13. Experimental study of Raman Effect

14. The experimental arrangement consists of source, filter, Raman tube and spectrograph. Light from mercury arc lamp (source of light) after passing through filter is allowed to incident on Raman tube. The Raman tube consists of 1 or 2 cm diameter and 10 to 15 cm long glass tube have flat surface on one side and horn shaped surface blackened outside on other side. The tube is surrounded by a water jacket for circulation of cool water to prevent from overheating of the tube. The experimental sample is placed in the tube scattered beam is examined by means of spectrograph

15. Classical Theory of Raman Effect When an electrically neutral molecule is kept in an uniform electrical field, it is polarized. The extent of polarization is given by the induced dipole moment μ= α . E α – Polarizability of the molecule A second degree tensor W h e n a r a d i a ti on of f r e q u e n cy ν i s f a ll i n g o n a molecule, the electrical field felt by the molecule is This electrical field polarizes the molecule and makes it to oscillate with the same frequency  

16. . substituting Eqn 2 in Eqn 1, we get Which is the associated induced dipole moment Such an oscillating dipole emits radiation of its own oscillation frequency, which is called Rayleigh Scattering  

17. Raman Scattering If the molecule undergoes rotation or vibration also, then the polarizability of it changes, hence the emitted frequencies also - Raman Scattering. Lets assume that the polarizability of the molecule α is changed by the vibrational frequency of the molecule sin 2 t Where, α is the equilibrium polarizability β is the rate of change of polarizability with vibration is the vibrational frequency  

18.   By expansion and using the relation Above equation shows that the oscillating dipole has three frequency components namely ν Rayleigh Line ( ν+ ) Raman Line ( Anti-Stokes) ( ν - ) Raman Line (Stokes)  

19. The Raman shift is depending on the β value. If the polarizability change with vibration is zero then the Raman shift is also zero, i.e. the vibration is not Raman active Therefore, the induced dipole moment oscillates with the following frequencies

20. Quantum theory of Raman effect

21. Applications of Raman Effect: To study the molecular structure of crystals and compounds. To know the number of atoms in a molecule, their relative arrangement, relative masses and chemical bonds between them To study the composition in plastics, mixtures To determine the type of Bond (single or double or triple bond) To study the spin and statistics of the nuclei To study the binding force between the atoms or group of atoms in crystals