Raman spectroscopy is an analytical technique that uses scattered light to measure the vibrational energy modes of a sample. It was discovered in 1928 by C.V. Raman and K.S. Krishnan, for which Raman won the 1930 Nobel Prize. Raman scattering occurs when light interacts with molecular vibrations, resulting in the energy of the scattered photons being shifted up or down. This provides information about the chemical structure and bonds within a sample. The shift is measured as a Raman spectrum that is unique for different materials.

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 and the color(wavelength) of the 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.

8. 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.

9. 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).

10. • 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.

11. • 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.

12. 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.

13. 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

14. Experimental study of Raman Effect

15. ▪ 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

17. 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 → (1)
α – Polarizability of the molecule A second degree tensor
• When a radiation of frequency ν is falling on a molecule, the electrical
field felt by the molecule is
• 𝐸 = 𝐸0 sin 2𝜋𝜗𝑡 → (2)
• This electrical field polarizes the molecule and makes it to oscillate with
the same frequency

18. • .
• substituting Eqn 2 in Eqn 1, we get
• 𝜇 = 𝛼𝐸0 sin 2𝜋𝜗𝑡 → (3)
• Which is the associated induced dipole moment
• Such an oscillating dipole emits radiation of its own
oscillation frequency, which is called Rayleigh Scattering

19. 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 𝜗𝑣𝑖𝑏
𝛼 = 𝛼0 + 𝛽sin 2𝜋𝜗𝑣𝑖𝑏t → (4)
• Where,
α0 is the equilibrium polarizability
β is the rate of change of polarizability with vibration
𝜗𝑣𝑖𝑏 is the vibrational frequency

20. 𝜇 = 𝛼𝐸 = (𝛼0 + 𝛽sin 2𝜋𝜗𝑣𝑖𝑏t) 𝐸0 sin 2𝜋𝜗𝑡 → (5)
By expansion and using the relation
𝑠𝑖𝑛𝐴𝑠𝑖𝑛𝐵 =
1
2
𝑐𝑜𝑠 𝐴 − 𝐵 − 𝑐𝑜𝑠 𝐴 + 𝐵
μ = 𝛼0𝐸0 sin 2𝜋𝜗𝑡 +
1
2
𝛽𝐸0 𝑐𝑜𝑠2𝜋 𝜗 − 𝜗𝑣𝑖𝑏 𝑡 − 𝑐𝑜𝑠2𝜋 𝜗 + 𝜗𝑣𝑖𝑏 𝑡
Above equation shows that the oscillating dipole has three frequency
components namely
-ν Rayleigh Line
-(ν+ 𝜗𝑣𝑖𝑏) Raman Line ( Anti-Stokes)
-(ν - 𝜗𝑣𝑖𝑏) Raman Line (Stokes)

21. • 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
• v v
, ,
    
 

23. Quantum theory of Raman effect

26. 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