This document provides an overview of Raman spectroscopy. It begins by defining spectroscopy as the study of how atoms and molecules interact with light. It then describes Raman scattering, which was discovered by C.V. Raman in 1928 and involves a change in frequency of scattered light that depends on the chemical structure of molecules. The rest of the document discusses key aspects of Raman spectroscopy such as Stokes and anti-Stokes scattering, the relationship between Raman and infrared spectroscopy, and applications of Raman spectroscopy such as molecular identification and quantification.

1. Raman Spectroscopy
Prof. V. Krishnakumar
Professor and Head
Department of Physics
Periyar University
Salem – 636 011, India

2. What is Spectroscopy?
• The study of how 'species' (i.e.,
atoms, molecules, solutions) react
to light. Some studies depend on
how much light an atom absorbs.
The electromagnetic radiation
absorbed, emitted or scattered by
the molecule is analyzed. Typically,
a beam of radiation from a source
such as a laser is passed through a
sample, and the radiation exiting
the sample is measured. Some, like
Raman, depend on a molecule's
vibrations in reaction to the light.

3. Light Scattering Phenomenon
• When radiation passes through a
transparent medium, the species present
in that medium scatter a fraction of the
beam in all directions.

4. Raman Effect (or Raman
Scattering)
• In 1928, the Indian physicist C. V. Raman
discovered that the visible wavelength of a small
fraction of the radiation scattered by certain
molecules differs from that of the incident beam.
• Furthermore, he noted that the change (shifts) in
frequency depend upon the chemical structure
of the molecules responsible for the scattering
First photographed Raman spectra

5. Why Raman?
• In Raman spectroscopy,
by varying the frequency
of the radiation, a
spectrum can be
produced, showing the
intensity of the exiting
radiation for each
frequency. This spectrum
will show which
frequencies of radiation
have been absorbed by
the molecule to raise it to
higher vibrational energy
states.

6. What Exactly Is Being Measured?
METHANE
When Light hits a
sample, It is Excited,
and is forced to vibrate
and move. It is these
vibrations which we are
measuring.

7. First Report of Raman Observation
Nature 121, 501-502 (31 March 1928)
A New Type of Secondary Radiation
C. V. RAMAN & K. S. KRISHNAN
Abstract
If we assume that the X-ray scattering of the ‘unmodified’ type observed by Prof.
Compton corresponds to the normal or average state of the atoms and molecules,
while the ‘modified’ scattering of altered wave-length corresponds to their
fluctuations from that state, it would follow that we should expect also in the case of
ordinary light two types of scattering, one determined by the normal optical
properties of the atoms or molecules, and another representing the effect of their
fluctuations from their normal state. It accordingly becomes necessary to test
whether this is actually the case. The experiments we have made have confirmed
this anticipation, and shown that in every case in which light is scattered by the
molecules in dust-free liquids or gases, the diffuse radiation of the ordinary kind,
having the same wave-length as the incident beam, is accompanied by a modified
scattered radiation of degraded frequency.

8. First Report of Raman Observation
Nature 121, 501-502 (31 March 1928)
A New Type of Secondary Radiation
C. V. RAMAN & K. S. KRISHNAN
Continue
The new type of light scattering discovered by us naturally requires very
powerful illumination for its observation. In our experiments, a beam of
sunlight was converged successively by a telescope objective of 18 cm.
aperture and 230 cm. focal length, and by a second lens was placed the
scattering material, which is either a liquid (carefully purified by repeated
distillation in vacuo) or its dust-free vapour. To detect the presence of a
modified scattered radiation, the method of complementary light-filters was
used. A blue-violet filter, when coupled with a yellow-green filter and placed
in the incident light, completely extinguished the track of the light through
the liquid or vapour. The reappearance of the track when the yellow filter is
transferred to a place between it and the observer's eye is proof of the
existence of a modified scattered radiation. Spectroscopic confirmation is
also available.

9. The Nobel Prize in Physics
1930
Professor Sir C.V. Raman
1888-1970
"for his work on the
scattering of light
and for the
discovery of the
effect named after
him"
http://nobelprize.org/nobel_prizes/physics/laureat
es/1930/raman-lecture.pdf

10. Rayleigh Scattering and Raman
Scattering
The frequency of the
scattered light can be:
• at the original
frequency (νI)
“Rayleigh scattering”
very strong.
• at some shifted
frequency
(νs = νI ± νmolecule) “Raman
scattering or Raman

11. Stokes and Anti-Stokes
Scattering
• Raman shift can correspond either to
rotational, vibrational or electronic
frequencies.
Δν = |νI – νs|
• Radiation scattering to the lower
frequency side (to the red side) of the
Rayleigh line is called Stokes scattering.
• Radiation scattering to the higher
frequency side (to the blue side) of the
Rayleigh line is called anti-Stokes

12. Stokes and Anti-Stokes
Scattering

13. Stokes and Anti-Stokes
Scattering

14. Number of bands in a Raman spectrum
As for an IR spectrum, the number of bands in the
Raman spectrum for an N-atom non-linear molecule is
seldom 3N-6, because:
polarizability change is zero or small for some
vibrations;
bands overlap;
combination or overtone bands are present;
Fermi resonances occur;
some vibrations are highly degenerate; etc…

15. Nature of Polarizability
Polarizability is the relative tendency of a charge distribution, like the
electron cloud of an atom or molecule, to be distorted from its normal
shape by an external electric field, which may be caused by the presence of
a nearby ion or dipole or by an applied external electric field.

16. Raman Activity of Molecular
Vibrations
• In order to be Raman active, a molecular rotation
or vibration must cause some change in a
component of the molecular polarizability. The
change can either be in the magnitude or the
direction of the polarizability ellipsoid.
• Polarizability ellipsoid is a three-dimensional body
generated by plotting 1/√α from the center of
gravity in all directions.
• This rule must be contrasted with that for IR
activity that requires change in the net dipole
moment of the molecule.

17. Raman
Activity of
H2O
Vibrations

18. Raman
Activity of
CO2
Vibrations

19. Raman and Infrared are Complementary
Techniques
• Interestingly, although they are based on two
distinct phenomena, the Raman scattering
spectrum and infrared absorption spectrum for a
given species often resemble one another quite
closely in terms of observed frequencies.
The infrared
and Raman
spectrum of
styrene/buta-diene
rubber.

20. Rule of Mutual Exclusion
• If a molecule has a center of symmetry,
then Raman active vibrations are infrared
inactive, and vice versa. If there is no
center of symmetry, then some (but not
necessarily all) may be both Raman and
infrared active.

22. Comparison between FT and
• FT-Raman
dispersive Raman
• Fluorescence-free Raman
spectra by 1064nm
excitation
• Simple measurement of
bulk samples due to
advantage of sample
compartment
• Quantification
• Dispersive Raman
• Better spatial resolution for
microscopy applications (down
to 1μm)
• Higher sensitivity and shorter
measurement times for non-fluorescing
samples
• Selection of different excitation
lines (488-785nm)

23. Uses of Raman Spectroscopy
Raman spectroscopy has become more widely used
since the advent of FT-Raman systems and remote
optical fibre sampling. Previous difficulties with
laser safety, stability and precision have largely
been overcome.
Basically, Raman spectroscopy is complementary
to IR spectroscopy, but the sampling is more
convenient, since glass containers may be used
and solids do not have to be mulled or pressed into
discs.

24. Applications of Raman spectroscopy
Qualitative tool for identifying molecules from their
vibrations, especially in conjunction with infrared
spectrometry.
Quantitative Raman measurements
a) not sensitive since Raman scattering is weak. But
resonance Raman spectra offer higher sensitivity,
e.g. fabric dyes studied at 30-50 ppb.
b) beset by difficulties in measuring relative
intensities of bands from different samples, due to
sample alignment, collection efficiency, laser power.
Overcome by using internal standard.

25. Raman vs IR spectroscopy
RAMAN IR
Sample preparation usually simpler
Liquid/ Solid samples must be free
from dust
Biological materials usually fluoresce,
masking scattering
Spectral measurements on vibrations Halide optics must be used-made
in the visible region-glass cells expensive, easily broken,
may be used water soluble
Depolarization studies are easily made IR spectrometers not usually
(laser radiation almost totally linearly equipped with polarizers
polarized)