When light interacts with matter, most photons are elastically scattered (Rayleigh scattering) but a small fraction may be inelastically scattered (Raman scattering). Raman scattering occurs when photons from the incident light are absorbed by a molecule, exciting it to a virtual energy state, and are re-emitted at a different wavelength. This change in wavelength provides vibrational, rotational, and other low-frequency information about the molecular structure. Raman spectroscopy utilizes monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. Scattered light from the sample is collected and its properties measured.

3. SPECTROSCOPY
• Spectroscopy is that branch of science which
deals with the study of interaction of
electromagnetic radiations with matter
• When the different types of electromagnetic
radiations are arranged in order of their
increasing wavelengths or decreasing frequencies,
the complete arrangement is called
electromagnetic spectrum.

5. Electromagnetic wave
One of the waves that are propagated by simultaneous periodic
variations of electric and magnetic field intensity and that
include radio waves, infrared, visible light, ultraviolet, X-rays,
and gamma rays

6. Concept of Polarisability in Raman Effect
• How molecule become polarised :
When a molecule interacts with electric field of
electromagnetic radiation, it undergoes distortion in its
electron cloud resulting in the formation of a dipole..
• Isotropically polarisable:
When spherically symmetrical molecule i.e. CH4, CCl4
etc. interacts with radiation, the electron forming the
bonds are displaced by an electric field symmetrically
in all directions. Thus molecule become polarised,
whatever will be the direction of applied electric field.

7. Anisotropically polarisable:
Non spherical molecules i.e. diatomic H2,
HCl, CO and linear polyatomic molecules CO2
,HC≡CH, etc. The electron forming the bonds
are more easily displaced by an electric field
applied in parallel or perpendicular to bond.

8. CLASSICAL THEORYOF RAMAN EFFECT
The induced dipole moment µ of polarized
molecule is given by:
P = αE ……(1)
α = polarisability of molecule
The electric field vector E is given by:
E = E0 sin 2πνt ……(2)
E0 = amplitude of oscillating electric field
ν = frequency of incident radiation
From (1) and (2)
P = αE0 sin 2πνt ……..(3)

9. As the molecule undergoes vibrational or rotational motion
with frequency νn ,the polarisabilty of the molecule changes
:
α =α0 + βsin2πνn t
α = equilibrium polarisabilty
β = rate of change of polarisabilty with vibration , then
eq. (3) can be written as:
P = (α0 + βsin2πνn t) (E0 sin 2πνt)
= α0E0 sin 2πνt + βE0 sin 2πνtsin2πνn t
P = α0E0 sin 2πνt +(1/2) βE0 {cos2π( ν-νn )t - cos2π( ν+νn )t}
Here, α0E0 sin 2πνt has frequency of incident radiation,
represents Rayleigh scattering . The (ν-νn ) represents
Stoke’s lines and (ν+νn ) represents anti Stoke’s lines

10. QUANTUM THEORY OF RAMAN EFFECT
hvi
• Molecule may deviate the photon without absorbing its energy
unmodified line.
• May absorb part of energy of incident photon modified Stokes
line.
• Molecule, itself being in an excited state, imparts some of its intrinsic
energy to incident photon modified Antistokes line.
Let a molecule of energy Ep be exposed to radiation (ν0) and after collision the
energy of molecule be Eq and the frequency of scattered photon be ν’.

11. From law of conservation of energy
hν0 + Ep + 1/2mv2 = hν’ + Eq + 1/2mv’2 …..(1)
Since, there is no change in temperature due to
collision so,
1/2mv2 =1/2mv’2
So equation (1) becomes
hν0 + Ep = hν’ + Eq
ν’ = ν0 + Ep- Eq/h
i. Ep = Eq, ν’=ν0 (unmodified line)
ii. Ep < Eq, ν’< ν0 (Stokes line)
iii. Ep > Eq, ν’> ν0 (Antistokes line)

12. What happens when light falls on a
material?

13. Rayleigh scattering is the scattering of light by the particles present in
the atmosphere. According to Rayleigh's scattering law, the amount of
scattering of the light is inversely proportional to the fourth power of
the wavelength. From the relation between scattering and wavelength,
we understand that shorter wavelengths scatter more. Since blue light
has a lesser wavelength than red light it scatters more.

16. • When a substance is irradiated
with monochromatic light, the light
scattered at right angles to the
incident light contained lines not
only of the incident frequency but
also of lower frequency and
sometimes of higher frequency as
well. The lines with lower
frequency are called Stokes' lines
whereas lines with higher
frequency are called anti-Stokes'
lines.

20. Types of Vibration

23. Instrumentation
There are following component involves:
1. Laser or source of light
2. Filter
3. Sample holder
4. Detector

25. 1. Laser or source of light
• Lasers are strong source strong to detect Raman
scattering.
• Lasers operate using the principle of stimulated
emission.
• Electronic population inversion is required.
• Population inversion is achieved by “pumping”
using lots of photons in a variety of laser gain
media

26. 2. Filter
For getting monochromatic radiations filters are
used.
They may be made of nickel oxide glass or quartz
glass.
Sometimes a suitable colored solution such as an
aqueous solution of iodine in CCl4 may be used as
a monochromator.

27. 3. Sample holder
For the study of raman effect the type of sample
holder to be used depends upon the intensity of sources
,the nature and availability of the sample.
The study of raman spectra of gases requires
samples holders which are bigger in size than those for
liquids.
Solids are powdered or dissolved before subjecting to
Raman spectrograph.
Any solvents which is suitable for the ultraviolet
spectra can be used for the study of Raman spectra.
Water is regarded as good solvents for the study of
compounds in Raman spectroscopy.

28. 4. Detector
Because of the weakness of a typical Raman signal.
Now days multichannel detectors like photodiode
arrays(PDA), charged couple devices(CCD) are
used.
Sensitivity & performance of modern CCD
detectors are high.

29. Selection Rule
• The polarisability of molecule must be change.
• For vibrational Raman spectra, the vibration must change
the polarisability of the molecule
∆v = ±1
where, ∆v = 1 corresponds to Stokes lines and
∆v = −1 corresponds toAnti-Stokes lines.
• For rotational Raman spectra, the polarisibilty of molecule
must be anisotropic.
∆J = 0, ±2
• All diatomic, linear and non spherical molecules are Raman
active.
• All spherical molecules are rotational Raman inactive. E.g
CH4 and CCl4 .

30. ADVANTAGES OF RAMAN OVER IR
•Raman lines are polarized.
•Raman spectra can be obtained even for molecules
such as O2, N2, CO2 etc. which have no permanent
dipole moment. Such a study has not been possible
by infra-red spectroscopy.
•Raman spectra can be obtained not only for gases
but even for liquids and solids whereas infra-red
spectra for liquids and solids are quite diffuse.

31. •Water can be used as solvent.
•Very suitable for biological samples in
native state (because water can be used as
solvent)
•Frequencies, spectrum is obtained using
visible light or NIR
•Glass and quartz lenses, cells, and optical
fibers can be used.
• Standard detectors can be used.

32. Disadvantages of Raman Spectroscopy
• Instrumentation is very expensive.
• Can not be used for metals or alloys.
• The Raman effect is very weak, so the detection
needs sensitive detectors.
• Fluorescense compounds when irradiated by the
laser beam, can hide the Raman spectrum.
• Sample heating through intenselaser
radiation can destroy the sample.

33. APPLICATIONS
• Application in inorganic chemistry
Vibrational spectroscopy still plays an important role
in inorganic systems. For example, some small reactive
molecules only exist in gas phase and XRD can only be
applied for solid state. Raman Spectroscopy is utilized to
study the structure of homonuclear diatomic molecules
(i.e. bond length) are all IR inactive

34. • Organic chemistry-
usеd to obtain information rеgarding thе prеsеncе or
absеncе of spеcific linkagеs in a molеculе, thе structurе
of simplе compounds, study of isomеrs (i.e. cis and
trans).
• Physical Chemistry-
helps to study physical chemistry concerning electrolytic
dissociation, hydrolysis and transition from crystalline
to amorphous state.

35. • Polymer Chemistry-
usеd for charactеrization of polymеr compounds,
by rеvеaling thе physical propеrtiеs and tacticity.
• Forensic science
including the identification of illicit drugs,
gunshot residue, inks used in explosives.

36. • Pharmaceutical Sector-
Raman Spectroscopy is widely adopted analytical technique in the
pharmaceutical market. . It can analyze excipients and active
pharmaceutical ingredients (APIs) of whole tablet within seconds,
to obtain an overview of the tablet through to a few tens of
micrometers to provide an in-depth analysis of individual grains
and phase boundaries.

37. • Biology-
• It helped to confirm the existence of low-
frequency phonons in proteins and DNA, and
their biological functions.
• used as a technique for, biochemical
characterization of wounds.
• used to detect cancer cells using bodily fluids
such as urine and blood samples