CLASSICAL THEORY OF RAMAN EFFECT.pptx

This presentation is about Classical theory of Raman Effect. This lecture gives brief explanation about rayleigh scattering and raman scattering and about the classical theory which talks about the polarisation of molecule and how the polarisation relates with raman scattering. Have fun Learning!

1. CLASSICAL THEORY OF RAMAN EFFECT 1 D. Sudha 21PGC29

2. INTRODUCTION Rayleigh Scattering Raman Scattering Hyper – Rayleigh Scattering Hyper – Raman Scattering Coherent Stokes - Raman Scattering Coherent anti Stokes – Raman Scattering 2

3. RAYLEIGH AND RAMAN SCATTERING Why SKY is blue ? RAYLEIGH SCATTERING (1871) Why SEA is blue ? RAMAN SCATTERING (1928) 3

4. RAYLEIGH SCATTERING When a radiation with frequency  is incident on system ( dust free, transparent gases and liquids, transparent solids etc.) , the light get scattered without a change in the frequency, known as Rayleigh Scattering. Before interaction, n photons of Energy h  Energy of molecule in its initial state – E i After interaction, Final state Energy of photon – ( n-1)h + h s Final State Energy, E f = E i  s =  i 4

5. RAMAN SCATTERING When a radiation with frequency  is incident on system it interacts with molecule and the light get scattered with a change in the frequency, known as Raman Scattering. Before interaction, n photons of Energy h  Energy of molecule in its initial state – E i After interaction, Final state Energy of photon – ( n-1)h + h s Final State Energy, E f = E i + h  fi  s =  s +  fi  s =  s -  fi 5

6.  Scattering at  is Rayleigh Scattering and    M is Raman Scattering 6

7. BLUE SKY Sunlight is scattered by the particles of the atmosphere. Smaller wavelength of light (Blue) scatter easier. Hence it is Blue in colour . At sunset and sunrise, the angle of sunlight changes and most of the blue and green (shorter) wavelengths of light get scattered. Hence we see more of the orange and red colours in the sky. 7 BLUE SEA The ocean is not blue because it reflects the sky, it appears blue due to its absorption of red light. Water molecules absorb some of the photons from the light and reflects the remaining  colours . In sea, there are many water molecules so it absorb all the red wavelengths and reflect blue. But is shallow water (like a drinking glass of water), there is not much water molecule, so it appears colourless .

8. CLASSICAL THEORY Based upon polarizability of molecules, which reflects how easy an electron cloud of a molecule can be distorted by an electric field (light). Helps to understand the concept basic to Spectroscopy. When a molecule is put into a static electric field it suffers some distortion, and creates an induced electric dipole moment and the molecule is said to be polarized.  Induced electric dipole moment  Polarisability of a molecule E  Electric Field 8

9. 9 DIATOMIC MOLECULE Example Homonuclear diatomic molecule H 2, Only one mode of vibration – Symmetric Stretching Placed in a Electric Field Anisotropy Polarisability creates Across the bond axis Along the bond axis

10. 10 Polarisability Ellipsoid Across the bond axis, Polarisability changes due to the Electric field. On Rotation, its polarisation is same in all direction. So Polarisability Ellipsoid is Circular Along the bond axis, Polarisability is greater so its cross section is less. Hence it is Elliptical. Raman Active This is same in heteronuclear diatomic ( HCl ) also, its Polarisability changes and hence Raman Active.

11. 11 POLYATOMIC MOLECULE In case of Polyatomic molecules, there is some internal vibrations which affects the size of the polarisability . When this type of molecule subjected to beam, its energy will be, Due to internal motion polarisability changes, is Equilibrium Polarisability β is rate of change of polarisability with vibrations

12. 12 On expanding, Here, if β = 0, dipole oscillates only at incident frequency Example, H 2 O Three Modes of Vibration Symmetric Stretching Asymmetric stretching Symmetric Bending

13. 13 Polarisability - ↑s Polarisability Ellipsoid size -↓s Polarisability - ↓s Polarisability Ellipsoid size - ↑s

14. 14 Size and Shape Approx. same Direction of major axis changes Linear Configuration with a Horizontal Axis Aprrox . Diatomic Molecule with Vertical axis

15. 15 Linear Molecule, CO 2 Three modes of fundamental vibration , Symmetric Stretching Asymmetric Stretching Bending

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17. 17 Chloroform CHCl 3 Expected to have polarizability surface as egg-shaped, fatter at the chlorine-containing end. But It is in ellipsoid form due to the symmetry axis.

18. 18 REFERENCE C.N. Banwell, Fundamentals of Molecular Spectroscopy, 4th edition, TMH, 1997. Derek A. Long, The Raman Effect: A Unified Treatment of the Theory of Raman Scattering by Molecules, John Wiley & Sons Ltd, 2002. Gerhard Herzberg, Infrared and Raman Spectra of Polyatomic Molecule, 1st edition, 1945.