Explain the Classical Theory of Raman Effect
Classical Theory of Raman Effect: Molecular Polarizability
The basic concept of classical theory of the Raman effect depends on the polarizability of a molecule and the applied Electric field.
When a molecule is placed in a static electric field, a distortion takes place in it because of the attraction of positively charged nuclei towards negative pole of the field and of electron towards positive pole. This separation of charge centers causes an induced electric dipole moment in the molecule and the molecule becomes polarized. The magnitude of induced dipole (𝜇) depends both on magnitude of the applied field (E) and on the case with which the molecule can be distorted. Thus,
𝜇 = 𝛼E
where α is the polarizability of the molecule.
In case of hydrogen molecule, the polarizibility is anisotropic i.e. the electron that form the bond are more easily
displaced by the field along the bond axis than along the one across this direction.
The polarizability of a molecule in various directions can be conventionally represented by polarizability ellipsoid as shown below-
The ellipsoid is a three-dimensional surface whose distance from the electrical centre of the molecule (in H2 this is also the centre of gravity) is proportional to 1/√𝛼i, where 𝛼i is the polarizability along the line joining a point 'i' on the ellipsoid with the electrical centre.
In case of diatomic molecule, the polarizibility in the same for all directions at right abgels to the bond axis, the ellipsoid has a circular cross section in this direction, so it is shaped like a tangerine. The shape of ellipsoid of all diatomic molecules like oxygen, HCl and of polyatomic linear molecule if similar to this except that the relative sizes of their major and minor axis are different.
When a diatomic molecules or a linear polyatomic molecule is subjected to a beam of radiation of frequency '𝜈' the electric field experienced by each molecule varies according to the equation-
E = Eo sin 2𝜋νt
and thus the induced dipole also undergoes oscillations of frequency '𝜈'.
𝜇 = 𝛼E
or, 𝜇 = 𝛼Eo sin 2𝜋𝜈t
Such an oscillating dipole gives out radiations of its own oscillation frequency and so it explains the Rayleigh scattering. We have not taken into consideration the vibration and rotation of molecule while deriving this equation. These also affect the polarizability of the molecule.
Effect of Vibration
If the molecule undergoes vibrational motion, the oscillating dipole will have superimposed upon it vibrational oscillation. The change in polarizibility due to a frequency of 𝜈vib can be expressed as-
𝛼 = 𝛼0 + β 2𝜋𝜈vibt
where, 𝛼0 is the equilibrium polarizability and β represents the rate of change of polarizability with the vibration.
Since, 𝜇 = 𝛼E
∴ 𝜇 = (𝛼0 + β 2𝜋𝜈vibt)E0 sin 2𝜋𝜈t
We know the trigonometric expression-
sin A sin B = ½[cos (𝐴 − 𝐵) − cos(𝐴 + 𝐵)]
𝜇 = (𝛼0 + 𝛽 sin 2𝜋𝜈vibt)E0 sin 2𝜋𝜈𝑡
𝜇 = 𝛼0E0 sin 2𝜋𝜈𝑡 + 𝛽 sin2𝜋𝜈vibt · E0 sin 2𝜋𝜈𝑡
𝜇 = 𝛼0E0 sin 2𝜋𝜈𝑡 + ½ 𝛽E0[cos 2𝜋(𝜈 − 𝜈vib)𝑡 − cos 2𝜋(𝜈 + 𝜈vib)𝑡]
So the oscillating dipole has frequency components 𝜈 ± 𝜈vib as well as exciting frequency 𝜈. So, the Raman shift will be equal to 𝜈vib.
Raman shift = ( 𝜈 + 𝜈vib − 𝜈) = 𝜈vib
Effect of Rotation
Again we consider a diatomic molecule, when such a molecule rotates its orientation varies with respect to the electric field of rotation. When such a molecule is not optically isotropic, the polariation will vary with time. In such a case the variation of '𝛼' can be expressed by the equation-
𝛼 = 𝛼0 + 𝛽 sin 2𝜋𝜈rott
where, 𝜈rot is the frequency of rotation.
This equation is identical with the equation obtained while considering the effect of vibration.
Substituting the above equation for dipole , i.e.
𝜇 = 𝛼E
𝜇 = 𝛼E0 sin 2𝜋𝜈𝑡 + &frac;12 𝛽'E0[cos 2𝜋(𝜈 − 2𝜈rot)𝑡 − cos 2𝜋(𝜈 + 𝜈rot)𝑡]
Thus the frequency of Raman lines will be (𝜈 + 2𝜈rot) and (𝜈 − 𝜈rot) and Raman shift will be 2𝜈. Thus Raman shift in this case will be twice the frequency of rotation of molecule.
Is should be clearly understood that if the vibration does not change polarizability of a molecule the 𝛽 = 0 and in such case dipole oscillates only at the frequency of the incident radiation. Similarly, if the rotation does not change polarizability of a molecule the 𝛽' = 0 and again the dipole oscillates only at the frequency of the incident radiation. Thus, we get the following general rules-
For a molecule to be raman active, the molecular rotation or vibration must produce some change in a component of molecular polarizability.
A change in molecular polarizability is reflected by a change in either the magnitude or the direction of the polarizability ellipsoid.
