Raman spectroscopy

By
Dr. Manohar Kakunuri

National Institute of Technology, Warangal

 RAMAN effect
Raman spectroscopy studies the inelastic scattering of light.
It was observed in 1928 by the Indian scientist Sir
Chandrasekhara Venkata Raman (physics Nobel price in
1930)
He led experiments on light scattering at Indian Association 1888-1970
for the Cultivation of Science (IACS) with K. S. Krishnan.

Photons are scattered by the interaction with vibrational and

rotational transitions in molecules. Raman spectroscopy is
used to study a material's chemical composition.

In Raman spectroscopy UV, VIS or NIR light is used as

radiation source, which has much higher energy than those
energy differences of vibration levels, so absorption of
photons is impossible.

Sir Kariamanikkam Srinivasa Krishnan,

 Raman Scattering
• The incident light will excite the system to a high-energy state.
When it recovers from this state, scattering reactions occur. The
elastically scattered light has the same energy as the incident light
- Rayleigh scattering.

• If the system gains energy during this process, the scattered light
loses this amount of energy and the system reaches a higher
energy state (higher energy level) than it had before - Stokes
scattering.
• If an already vibrating system is excited, it loses energy during
this process and the system reaches a lower energy level than it
had before - Anti-Stokes scattering.
 What is Raman Scattering?
The Raman scattering technique is vibrational molecular spectroscopy
that derives from an inelastic light scattering process.

The Raman effect arises when a photon is incident on a molecule and

interacts with the electric dipole of the molecule.
 What can happen if photon interacts with molecule
Can be absorbed / Can be absorbed and re-emitted at a different wavelength
(electrons reaches 1st electronic exited state) Can Scattered (virtual state)
1 st electronic
exited state

Virtual state

Anti-stokes luminescence
stokes
Vibrational levels
Rayleigh

IR
Raman scattering:
Atom in its ground state absorbs a photon(laser light) which does not allow
it to reach a true electronic excited state (with a noticeable lifetime). The electron
does not relax in the virtual state because there is no excited state.
Absorption occurs when the source has enough energy to bring the molecule into an
excited electronic state. The resulting observed light radiating from the sample will
be in the form of luminescence.
 Raman Spectroscopy
Raman effect is a 2-photon scattering process
These processes are inelastic scattering:
Stokes scattering: energy lost by photon:
 —  (( —  )) 
Photon in Photon out
No vibration Vibration
Anti-Stokes scattering: energy gained by photon:
(( —  )) —  
Photon in Photon out
Vibration No vibration
But dominant process is elastic scattering:
Rayleigh scattering
 —  —  
Photon in Photon out
No vibration No vibration
If incident photon energy E; vibration energy v, then
in terms of energy, photon out has energy:
E-v Stokes scattering
E+v anti-Stokes scattering
E Rayleigh scattering
 Advantages of Raman Spectrum
• Minimal or no sample preparation
• Non-destructive analysis, so the same sample can be used in other
analyses
• A Raman spectrum plots light intensity (unit, e.g., counts, counts
per second or arbitrary units) versus light frequency (relative
wavenumbers).

• Generally, only the Stokes bands are recorded, because they are
more easily detectable due to their higher intensity. The Rayleigh
line equals 0 Raman shift, so that Anti-Stokes lines have negative
wavenumbers and Stokes lines have positive wavenumbers. The
wavenumber shift is characteristic for a material.
 Raman chart
Raman shift= λ-1incident - λ-1scattered
 Polarizability
The polarizability is a measure for the electron cloud's ability to deform in
contrast to the atomic nuclei.

When placed into an electric (or oscillating) field the electrons are pulled
towards the positive charge and the atomic nuclei towards the negative charge,
which induces a dipole moment and results in scattering reactions.

The change of polarizability depends on the molecule geometry. For example,

during the symmetric stretching vibration of the linear CO2 molecule, the
polarizability gets smaller during the stretching as opposed to the compression.
It changes, and the vibration is Raman-active (but IR-inactive). During the
asymmetric stretching vibration, on the other hand, the polarizability does not
change, and the vibration is Raman-inactive (but IR-active).
 Selection rule for Raman spectrum
• Vibration is active if it has a change in polarizability, .

• Polarizability is the ease of distortion of a bond.

• For only Raman-active vibrations, the incident radiation does not

cause any change in the dipole moment of the molecule, but instead a
change in polarizability.
Example: There are 2 normal modes of CO2. Only 1 is Raman active

 is dipole moment;
 is polarizability
Examples
 Raman Spectrum of Graphene and Graphene Layers (Defect free
graphite)

Wavelength of laser= 514

nm

The two most intense features are the G peak at 1580 cm-1 and a G’ peak at
2700 cm-1.
Absence D-peak around 1350 cm-1 because of defect free graphite sample.
,
Comparison of 2D peak of graphene and graphite.

Thus Raman spectroscopy

can clearly distinguish a
single layer, from a bilayer
from few (lessthan 5) layers.

Graphene Graphite
The 2D peak consists of two single, sharp 2D peak in graphene
components 2D1 and 2D2.
Raman shift of G’ peak increases with number of layers
 Raman spectrum of the the amorphous graphite sample

Amorphous graphite
sample

Crystalline graphite

Y. Raitses et al., Journal of Nuclear Materials 375 (2008)

365–369
 A comparative study of single-walled carbon nanotube
purification techniques using Raman spectroscopy
Single-walled carbon nanotubes (SWCNTs) synthesized by the chemical vapour deposition
(CVD) technique.
Removed impurities using both hydrochloric acid treatment and surfactant purification
 Typical spectrum of unpurified single-walled carbon nanotubes

Y. Raitses et al.,
Journal of Nuclear
Materials 375
Features of different bands (2008) 365–369

The G+ represents atomic displacement along the tube axis.

The G− refers to modes with atomic displacement in the
circumferential direction
The lowest energy feature is the radial breathing mode (RBM),
which appears at 168cm−1 and corresponds to the vibration of
carbon atoms in the radial direction.
 Fourier Transform Infrared Spectroscopy
FT-IR stands for Fourier Transform Infra Red. In infrared spectroscopy, IR radiation is
passed through a sample.
FTIR is a tool that uses to identify the functional groups of compounds.
Some of the infrared radiation is absorbed by the sample, and some of it is passed through
(transmitted). The resulting spectrum represents the molecular absorption and transmission,
creating a molecular fingerprint of the sample. Like a fingerprint, no two unique molecular
structures produce the same infrared spectrum.

Within chemical molecules, the chemical bonds are constantly vibrating (stretching /
bending) by absorbing energy from the surroundings.
The energy associated with these events is relatively small. The energy of the vibrations
coincides with the IR part of the electromagnetic spectrum.

Photon energy associated with the IR part of the spectrum is not enough to excite electrons
but may induce vibrational excitations of covalently bonded atoms.
IR spectrometer
A light source emits polychromatic IR light, which is
focused on a sample.

The light is partially absorbed by the sample when it is

passing through it.

Molecules in the sample interact with the light, they take up

energy and use this energy to vibrate, with the condition that
the dipole moment changes.

A detector registers how much light is transmitted through

the sample. The result is a characteristic spectrum showing
the transmittance (absorbance) of electromagnetic radiation
as function of wavelength (wavenumber).
 IR
sample Detector
source
IR
% of transmittance

X units
of
trough energy

Wave number cm-1

The Sample Analysis Process
 The Sample Analysis Process
The normal instrumental process is as follows:
1. The Source: Infrared energy is emitted from a glowing black-body source. This beam
passes through an aperture that controls the amount of energy presented to the sample
(and, ultimately, to the detector).
2. The Interferometer: The beam enters the interferometer where the “spectral encoding”
takes place. The resulting interferogram signal then exits the interferometer.
3. The Sample: The beam enters the sample compartment where it is transmitted through
or reflected off of the surface of the sample, depending on the type of analysis being
accomplished. This is where specific frequencies of energy, which are uniquely
characteristic of the sample, are absorbed.
4. The Detector: The beam finally passes to the detector for final measurement. The
detectors used are specially designed to measure the special interferogram signal.
5. The Computer: The measured signal is digitized and sent to the computer where the
Fourier transformation takes place. The final infrared spectrum is then presented to the
user for interpretation and any further manipulation.
 Selection rule for IR
Vibrations are IR active only if dipole movement changes with
vibration
A diatomic molecule with the same atom can’t be exited to vibrate
because no dipole movement is present.
If dipole movement is not present in beginning it can be induced due
to asymmetric displacement of the center of the charge.

Number of vibrations in nonlinear molecule 3N-6

Linear molecule 3N-5
 Algorithm for FTIR data interpretation
Two absorptions near 1810 and 1760 cm-

Anhydride
Broad absorption near 2400-3500 cm-
Acid

Medium absorption near 3500 cm-

amide
Two week absorptions near 2850 & 2750 cm-

aldehyde
Strong absorption near 1150 cm-

Ester
Ketone
1. Check for carbonyl group: 1715 cm-
2. Check for alchol-0H: strong broad peak near 300-3300, N-H medium broad peak
3000-3300 cm-
3. Check for triple bonds C≡C band near 1800cm-, C≡N band near 2000cm-
4. Check for C-H for 2700-3000 cm-
IR Chart
 Comparison of Raman and IR Spectroscopy
Mutual exclusion principle
In a molecule with a center of symmetry it is seen that vibrations that are
Raman active are IR inactive and vice-versa, this is called the Principle of
mutual exclusion

In molecules with different elements of symmetry, certain bands may be

active in IR, Raman, both. For a complex molecule that has no symmetry
except identity element, all of the normal modes are active in both IR and
Raman.
In general, the strong bands in the IR spectrum of a compound corresponds to
weak bands in the Raman and vice versa. This complimentary nature is due to
the electrical characteristic of the vibration. If a bond is strongly polarised, a
small change in its length such as that occurs during a vibration, will have
only a small additional effect on polarisation. Vibrations involving polar bonds
(C-O , N-O , O-H ) are, therefore, comparatively weak Raman scatterers. Such
polarised bonds, however, carry their charges during the vibrational motion, (
unless neutralised by symmetry factors), which results in a large net dipole
moment change and produce strong IR absorption band. Conversely, relatively
neutral bonds ( C-C , C-H , C=C ,) suffer large changes in polarisability during
a vibration, though this is less easy to visualise. But the dipole moment is not
similarly affected and vibrations that predominantly involve this type of bond
are strong Raman scatterers but weak in the IR.
Sl. No Raman IR
1 It is due to the scattering of
It is the result of absorption of
light by the vibrating
light by vibrating molecules.
molecules.
2 The vibration is Raman The vibration is IR active if there
active if it causes a change
is a change in dipole moment
in polarisability. during the vibration.
3 The vibration concerned should
The molecule need not have a
possess a permanent dipole change in dipole moment due to
moment. that vibration.

4 Water can be used as a Water cannot be used due to its

solvent. intense absorption.
5 Sample preparation is not Sample preparation is elaborate
very elaborate sample can Gaseous samples can rarely be
be almost in any state. used.
6 symmetric=Raman active asymmetric=IR active