4. Sir Chandrasekhara Venkata Raman
November 7, 1888 – November 21, 1970
Carried out ground-breaking work in the field of light
scattering, which earned him the 1930 Nobel prize for
Physics
• Discovered the “Raman effect”.
• In 1954, India honoured him with its highest
civilian award, the Bharat Ratna.
Also independent observations by
Grigory Landsberg and Leonid Mandelstam.
5. • Raman Spectroscopy is a non-destructive chemical analysis technique
which is used to analyze vibrational, rotational, and other low-
frequency modes in a system, providing information about chemical
structure, crystallinity and molecular interactions.
• When a monochromatic radiation of definite frequency (ʋ ) is passed
through a substance, the light is scattered at right angles to the incident
radiation containing lines of-
i. Incident frequencies
ii. Certain discrete frequencies above or below that of incident
radiation..
• The transmitted lines with same frequency as that of incident radiation
is called Rayleigh scatter.
• However a small amount of light (typically 0.0000001) scattered at
different frequency (or wavelength) is called Raman Scatter.
7. Theory of Raman spectra.
When a monochromatic beam is passed through a scatterer (liquid or gas), small
fraction of light is scattered due to collision between molecules of substance and
photons of light. Two cases may arise depending upon whether a collision between
a photon and molecules in it’s ground state is elastic or inelastic in nature.
Case 1- if the collision is elastic – this lead to the appearance of unmodified lines
(or unmodified frequency of light) in the scattered beam and this explain Rayleigh
scattering.
Case 2 - if the collision is inelastic – there will be exchange or transfer of energy
between the scattering molecules and the incident photon leading to the Raman
scattering with the two cases.-
1. the molecule absorbs energy from the incident photon and reaches a higher
rotational vibrational state after excitation and emission of photon. Thus the
energy of emitted photon becomes less, hence frequency of scattering lines is
less than that of incident light. This gives stroke's line of Raman spectra.
2. the molecule imparts some of its intrinsic energy to incident photon ,the
emitted photon has higher energy which corresponds to anti stokes line of
raman spectra where frequency of scattering lines is more than that of incident
light. 7
8. Energy Scheme for Photon Scattering
Rayleigh
Scattering
(elastic)
Stokes
Scattering
Anti-Stokes
Scattering
h 0
h 0
h 0 h
h
0
m
h 0+h
m
E0+h
E0
m
Raman
(inelastic)
The Raman effect comprises a very small fraction,
about 1 in 107 of the incidentphotons.
Virtual
State
IR
Absorption
Energy
9. Stokes lines Anti Stokes lines
• The frequency of scattered lines is
more than that of incident light.
• Caused by molecules at higher
energy level which are less
populated.
• These are less intense with low
intensity of absorption.
• At high temperature, molecules
are raised to higher energy state,
thus these gradually increases and
become prominent.
• The frequency of scattered lines
is less than that of incident light
• Caused by molecule at lower
energy level which is more
populated.
• These are more intense with high
intensity of absorption.
• At low temperature, these takes
place more frequently.
Stokes Vs Anti Stoke’s lines
10. The difference between the frequency of the incident light
and that of scattered light is constant and it depends only
on the nature of substance. It is completely independent of
the frequency of incident light.
If ʋ o is the frequency of incident light and ʋ r is the
frequency of scattered light, then
θʋ = [ ʋ o - ʋ r }
This difference is called the Raman shift.
The various observations made by Raman are called Raman
effect and the spectrum obtained is called Raman spectrum.
RAMAN EFFECT
12. Mutual Exclusion Principle
Z
Symmetric molecules Based on polarisability
O = C = O O = C = O
Raman active
IR inactive
Raman inactive
IR active
According to the principle, ‘”All vibrations of a
molecule, with a centre of symmetry, which are
Raman active are infra-red inactive and vice versa”.
+ - +
O = C = O
Raman inactive
IR active
13. Differences between IR and Raman
methodsS.NO
Raman spectra IR spectra
01 It is due to the scattering of light
by the vibrating molecules.
It is the result of absorption of
light by vibrating molecules.
02 The vibration in Raman is active if
it causes a change in polarisability.
Vibration is IR active if there is
change in dipole moment.
03 Gives an indication of covalent
character in the molecule.
Gives an indication of ionic
character in the molecule.
04 Water can be used as a solvent. Water cannot be used due to it
is opaque to IR.
05 Can be obtained for a compound in
all three states.
IR spectra is quite diffused in
liquid and solid state.
14. Applications of Raman
spectroscopy
• Raman spectroscopy is commonly used in chemistry, since
vibrational information is specific to the chemical bonds and
symmetry of molecules.Therefore, it provides a fingerprint by
which the molecule can be identified.
• Raman spectroscopy is helps in studying the structure of
molecule and also the structural changes which occur due to
association, dissociation and solvation, study the kinetics of fast
reactions.
SOMEOFTHE IMPORTANTAPPLICATIONS ARE-
Elucidation of molecular structure
Nature of chemical bond
Quantitative analysis of mixture
Mechanism of tautomerism
15. Applications of Raman
spectroscopyCarbon Materials
Purity of CNTs
Specifying sp2 and sp3 structure in
carbon materials
Geology and Mineralogy
Gemstone and mineral identification
Fluid inclusions
Mineral and phase distribution in rock
sections
Pharmaceuticals
Compound distribution in tablets
Polymorphic forms
Contaminant identification
Life Sciences
Bio-compatibility
DNA/RNA analysis
Drug/cell interactions
Disease diagnosis
