Lect. 21 raman spectroscopy introduction
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The document provides a detailed overview of Raman Spectroscopy, including its principles, conditions for Raman active molecules, and the significance of the Raman effect in studying molecular transitions. It discusses the differences between Raman and other spectroscopies like microwave and IR spectroscopy, and highlights the characteristics of Raman spectra and their applications. The document also emphasizes the historical context and contributions of key figures in the development of Raman Spectroscopy.
Lect. 21 raman spectroscopy introduction
- 1. Dr. Y. S. THAKARE M.Sc. (CHE) Ph D, NET, SET Assistant Professor in Chemistry, Shri Shivaji Science College, Amravati Email: yogitathakare_2007@rediffmail.com B Sc- III Year SEM-V PAPER-III PHYSICAL CHEMISTRY UNIT- VI Raman Spectroscopy 24-November -20 1
- 3. CONTENTS Introduction About energy level diagram and study of Microwave, IR spectroscopy. Need of studying Raman Spectroscopy. Principle Conditions for Raman active molecules Details about Raman effect Raman spectra and its characteristics Rotational Raman spectra Vibrational Raman spectra Applications References
- 4. INTRODUCTION Raman effect, an adjunct to electronic and infrared spectra was predicted by Adolf Smekal (1923) and discovered by C. V. Raman and his co-worker K. S. Krishnan in 1928. For his excellent works, C. V. Raman won Nobel Prize in 1930. Raman spectroscopic measurement is usually carried out in the visible region(400-700 nm). Raman spectra relate to vibrational/rotational transitions in molecules but in a different manner. In this case only scattering is measured but not absorption of radiation.
- 6. About Energy Levels The branch of spectroscopy can be studied under two following heads 1) Atomic spectroscopy 2) Molecular spectroscopy In Molecular spectroscopy, Electronic levels are accompanied by number of vibrational levels and similarly vibrational levels are accompanied by number of rotational levels.
- 7. What is Microwave and IR Spectroscopy? The transition of electron from one rotational level to another rotational level within same vibrational level is known as Rotational Spectra. And such type of radiations are available in microwave region hence rotational spectroscopy is also known as Microwave Spectroscopy. The transition of electron from one vibrational level to another vibrational level within same electronic level is known as Vibrational Spectra. And such type of radiations are available in IR region hence Vibrational Spectroscopy is also known as IR Spectroscopy. To obtain such spectra the molecule must be in gaseous state and must posses permanent dipole moment.
- 8. Need of studying Raman Spectroscopy As we know that, molecule with no permanent diople moment would have no pure rotational spectrum, and the molecules which do not have vibrational oscillation would have no IR absorption or emission spectra. Raman spectroscopy allows us to determine rotational and vibrational level spacing for such systems, and hence to determine bond lengths and force constants for such molecules. That is we can use Raman Spectrum to study H2,O2,N2………etc
- 9. What happens when light falls on a material? Transmission Reflection Absorption Luminescence Elastic Scattering Inelastic Scattering
- 10. Raman Spectroscopy 1 in 107 photons is scattered inelastically Infrared (absorption) Raman (scattering) v” = 0 v” = 1 virtual state Excitation Scattered Rotational Raman Vibrational Raman Electronic Raman
- 11. Structure of CO2 molecule The modes of vibrations of CO2 molecule are represented as below: The remaining three vibrations are IR active because that produce changes in dipole moment of CO2 molecule. The two bending vibrations have same energy, because they are degenerate (doubly). So, IR spectrum of CO2 molecule shows two absorption bands.
- 12. Principle of Raman Effect When a monochromatic beam of light is allowed to pass through a substance in solids, liquids or gaseous state, the light is scattered which is perpendicular to the incident radiation and contain some additional frequencies over and above that of
- 13. When a monochromatic beam of light was allowed to pass through a substance in the solid liquid or gaseous state the light is scattered and contains some additional frequencies over and above that of the incident frequency. This phenomenon is called as Raman Effect The lines having lower frequency that that of incident frequency are called as stroke line and those having higher frequency are called as anti-stroke lines Line with the same frequency as the incident light is called as Rayleigh lines. Raman spectroscopy:
- 16. If νi is the wave number (frequency) of incident radiation and νs that of the radiation scattered by the given molecule, then the Raman shift or Raman frequency (∆ν) is given by ∆ν = νi − νs For stokes lines, ∆ν = +ve i.e. νi > νs or νs < νi For Anti-stokes lines, ∆ν = −ve i.e. νi < νs or νs > νi For Rayleigh lines ∆ν = 0 i.e. νi = νs Thus, the Raman frequencies observed for a particular substance are characteristic of that substance . The various observation made by Raman are called Raman effect and the spectrum observed is called as Raman spectrum. Raman spectrum is shown below (Fig 1))
- 19. TRANSITION OF RAMAN AND RAYLEIGH SCATTERING. Elastic collision- rayleigh scattering (No change in energy) Inelastic collision- stokes scattering and antistokes scattering. (Exchange of energy)
- 20. Raman shift (∆ν) generally lies within the range falls in far and near infrared region of the spectrum. This leads to conclusion that the change in energy of the scattered light in Raman effect corresponds to the energy changes accompanying rotational and vibrational transition in the molecule.
- 21. Characteristics of Raman spectrum The lines observed in Raman effect exhibit number of characteristics which are given below. The intensity and width of Raman wing vary from liquid to liquid and depends upon the anisotropy of the molecule of the liquid i) The intensity of stokes lines is always greater than the Anti- stokes lines. ii) Raman shift (∆ν) , generally lies within the far and near infrared regions. iii) Raman lines are symmetrically displaced about Rayleigh line. iv) Raman lines are due to change in polarization of the molecules.
- 22. Characteristics of Raman spectrum The intensity of stokes lines is always greater than the antistokes lines. Raman shift, Δν generally lies within the far and near infrared regions. Raman lines are symmetrically displaced about Rayleigh line. Raman lines are due to change in polarization of the molecules. . If νi is the frequency of incident radiation and νs is of radiation scattered by the given molecule, then the Raman shift is given by, νi = νs Rayleigh line νi > νs Stokes line νi < νs Anti Stokes line For stokes line: Δν =+ve i.e νi > νs For antistokes line: Δν = -ve i.e νi < νs
- 23. Conditions for Raman active molecules For a molecule to be Raman Active, vibrations/ rotation should be accompanied by change in polarization of the molecule. And that polarized molecules are effective at certain wavelength which causes scattering of light.