Raman Spectroscopy - Principle, Criteria, Instrumentation and Applications

Basic principle of Raman scattering- Difference between Rayleigh and Raman Scattering- Major criteria for Raman active in compounds,-Stroke's lines and Anti-stoke lines- Difference and between IR and Raman spectroscopy- Wide applications of Raman spectroscopy.

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1. TECHNIQUES AND INSTRUMENTATION OF RAMAN SPECTROSCOPY N . PRABHA , I M.Sc., Chemistry

2. RAMAN SPECTROSCOPY A technique in which difference in frequency of incident and scatter radiation is studied It was discovered by sir c. v. raman He got nobel prize in 1930 In crystals, grigory landsberg and leonoid mandelstam

3. RAYLEIGH SCATTERING AND RAMAM SCATTERING RAYLEIGH SCATTERING When a monochromatic light is allowed to interact with molecule, a portion of monochromatic light is scattered If the frequency of scattered light is equal to the frequency of incident light is called Rayleigh scattering They are elastic scattering

4. RAMAN SCATTERING If the frequency of scattering light is not equal to the frequency of incident light is called raman scattering They are inelastic scattering Corresponding spectral line is called raman lines.

5. STOKES AND ANITSTOKES LINES

6. STOKES LINES When a molecule interact with the monochromatic light. Apart of light is scattered. Some amount of scattered light frequency is less than the frequency of incident light γ R= γ i -∆ γ γ R= raman spectroscopy Γ i =incident frequency ∆ γ raman shift (∆ γ = γ s- γ i ) Lines which are seen at the frequency less than the incident They are more intense It results in absorption process Raman frequency carries positive sign

7. ANTISTOKE’S LINE Some amount of the scattered light frequency is greater than frequency of incident light γ R= γ i +∆ γ Lines which are seen at the frequency greater than incident They are less intense It results in emission process Raman frequency carries- ve sign

8. CRITERIA FOR RAMAN SPECTRA Change in polarisability of the molecule during vibration Selection rule ∆v=±1, ∆J=0 or ∆J=±2

9. INSTRUMENTATION

10. SOURCE Lasers, which are highly monochromatic coherent beam. They are extremely powerful E.g rare gas lasers ( Ar -Kr etc) which can provide intencites as great as one million times that of sunlight. Previous raman measurements were taken with mercury arc.

11. SAMPLE CELL Where the sample is placed in narrow glass or quartz tube COLLECTING LENS Light scattered from the sample are collected MONOCHROMATOR Where the scattered radiations are resolved by grating DETECTOR It contains phototubes of photomultiplier which amplifies the signal

12. IR SPECTRA Obtained as a result of absorption of light Condition for a vibration to be IR active is that molecule should possess change in dipole moment Since water absorbs IR radiations, it cannot used as a solvent Dil. Soln of small amount of sample is enough to take IR spectrum RAMAN SPECTRA Obserced due to scattering of light Change in polarisibility determines the vibration for raman active Water can be used as solvent Concentrated solutions are to be used in order to get intense bands

13. Photochemical reactions do not takes place as IR is less energetic Optical system made up of alkali halides It is both accurate and sensitive Sample may be colored or colorless It cannot be recorded on one exposure it needs two runs Photochemical reactions takes place sometimes optical systems are made up of glass or quartz It is very accurate but less sensitive Sample should be perfectly colorless and dust free One exposure is enough to get spectrum

14. APPLICATIONS Used as structural diagnosis. Widely used in deciding the constitution of organic compounds. Used to identity linear and non-linear structural similarities Used to study the strength and nature of forces which are present in crystals Calculation of force constants, moment of inertia Used in study of complex compounds, mixed molecules and water of crystalisation Used in study of structure of organic compounds and isomerism used in study of amorphous states and crystalline states etc