This document provides an overview of Raman spectroscopy. It begins with an introduction, explaining that Raman spectroscopy involves measuring the wavelength and intensity of inelastically scattered light from molecules. This scattered light occurs at shifted wavelengths corresponding to molecular vibrations. It then provides a brief history of Raman spectroscopy and its development. The document outlines some key aspects of Raman spectroscopy, including that it is a vibrational spectroscopy that is complementary to infrared spectroscopy. Raman spectroscopy can be used to study samples with minimal preparation across various physical states. The remainder of the document discusses various technical aspects of Raman spectroscopy in more detail, including classical theories, instrumentation components like lasers and filters, and conditions required for Raman scattering to occur. It provides examples

1. Raman spectroscopy
NITISH KUMAR
M.PHARM (ANALYSIS)
2015-2016
GT Road (NH-95), Ghal Kalan, Moga(142001), Punjab, India

2. INTRODUCTION
• Raman spectroscopy is the measurement of the
wavelength and intensity of inelastically scattered
light from molecules. The Raman scattered light
occurs at wavelengths that are shifted from the
incident light by the energies of molecular
vibrations.
• Raman spectroscopy is used to determine the
molecular motions, especially the vibrational
one.
2

3. Time lap
• 1923 – Inelastic light scattering is predicted by A. Smekel
• 1928 – Landsberg and Mandelstam see unexpected frequency shifts in
scattering from quartz
• 1928 – C.V. Raman and K.S. Krishnan see “feeble fluorescence” from
neat solvents
• 1930 – C.V. Raman wins Nobel Prize in Physics
• 1961 – Invention of laser makes Raman experiments reasonable
• 1977 – Surface-enhanced Raman scattering (SERS) is discovered
• 1997 – Single molecule SERS is possible
3

4. OVERVIEW
• A vibrational spectroscopy
- IR and Raman are the most common vibrational spectroscopes for
assessing molecular motion and fingerprinting species
- Based on inelastic scattering of a monochromatic excitation source
- Routine energy range: 200 - 4000 cm–1
• Complementary selection rules to IR spectroscopy
- Selection rules dictate which molecular vibrations are probed
- Some vibrational modes are both IR and Raman active
• Great for many real-world samples
- Minimal sample preparation (gas, liquid, solid)
- Compatible with wet samples and normal ambient
- Achilles Heal is sample fluorescence
4

5. Raman spectrometer’s mechanism.
When a substances (in any state) is irradiated with a monochromatic light
of definite frequency(v),the light scattered at right angle to the incident
light contains lines of 1. Incident frequency and 2.Also of lower
frequency
Sometimes lines of higher frequency are also obtained that of the incident
beam will be scattered. It is called Raman scattering.
The line with lower frequency are called Stoke’s lines.
Also, the line with higher frequency are called Antistoke’s lines.
The line with the same frequency as that of the incident light is called
Rayleigh line.
5

6. Frequency :-
This difference is called Raman frequency or Raman shift.
6

7. .

8. • It may be noted that raman frequencies for a
particular substances are characteristic of that
substances.
• The various observation made by raman are
called raman effect.
• Also the spectrum obtained is called raman
spectrum.
8

9. Classical theory of raman effect
• According to the classical theory of electromagnetic radiation, electric and magnetic fields
oscillating at a given frequency are able to give out electromagnetic radiation of the same
frequency. One could use electromagnetic radiation theory to explain light scattering
phenomena.
• For a majority of systems, only an induced electric dipole moment μ is taken into
consideration. This dipole moment which is induced by the electric field E could be
expressed by the power series
μ=μ(1)+μ(2)+μ(3)+⋯
where
μ(1)=α⋅E
μ(2)=12β⋅EE
μ(3)=16γ⋅EEE
α is termed the polarizability tensor.
It is a second-rank tensor with all the components in the unit of CV-1m2. Typically, orders of
magnitude for components in α, β, and γ are as follows,
α, 10-40 CV-1m2;
β, 10-50 CV-2m3; and
γ, 10-61 CV-3m4.
According to the values, the contributions of μ(2)and μ(3) are quite small unless electric field is
very high. Since Rayleigh and Raman scattering are observed quite readily with very much
lower electric field intensities, one may expect to explain Rayleigh and Raman scattering in
terms of μ(1) only.
9

10. Classical theory of raman effect
y of Raman Effect
Colthup et al., Introduction to Infrared and Raman Spectroscopy, 3rd ed., Academic Press, Boston: 1990
mind = aE
polarizability
10

11. .
Electronic
Ground State
1st Electronic
Excited State
ExcitationEnergy,s(cm–1)
Vib.
states
4,000
25,000
0
fluorescence
IR
s
s semit
2nd Electronic
Excited State
Raman
∆s=semit-s
s ∆s
fluorescence
Impurity
Fluorescence
= Trouble
Raman Spectroscopy: Absorption, Scattering, and Fluorescence
Stokes Anti-Stokes
11

12. . Raman Spectroscopy: Classical Treatment
• Number of peaks related to degrees of freedom
DoF = 3N - 6 (bent) or 3N - 5 (linear) for N atoms
• Energy related to harmonic oscillator
• Selection rules related to symmetry
Rule of thumb: symmetric=Raman active, asymmetric=IR active
Raman: 1335 cm–1
IR: 2349 cm–1
IR: 667 cm–1
CO2

s or s 
c
2
k(m1  m2)
m1m2
Raman + IR: 3657 cm–1
Raman + IR: 3756 cm–1
Raman + IR: 1594 cm–1
H2O
12

13. Theory of raman spectra
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.
The frequency of scattered light and the incident photon which is either higher or lower
than that of the incident photon is called raman frequency.
Totalenergybeforecollision=totalenergyaftercollision
13

14. Presentation of Raman Spectra
lex = 1064 nm = 9399 cm-1
Breathing mode:
9399 – 992 = 8407 cm-1
Stretching mode:
9399 – 3063 = 6336 cm-1
14

15. Rayleigh Scattering:- Occurs when incident EM radiation induces an
oscillating dipole in a molecules, which is re-radiated at the same frequency.
Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
•Elastic (l does not change)
•Random direction of emission
•Little energy loss
•of emission
•Little energy loss
4 2 2
0
4 2
8 ( ') (1 cos )
( )sc
E
E
d

 a 
l


15

16. Raman Scattering
Occurs when monochromatic light is scattered light has been weakly modulated
by the characteristic frequencies of the molecules.
Raman spectroscopy measures the differences between the wavelengths of the
incident radiation and the scatted radiation.
max 0
max max 0
max max 0
( ) cos2
1
cos2 ( )
2
1
cos2 ( )
2
equil
z zz
zz
vib
zz
vib
t E t
d
r E t
dr
d
r E t
dr
m a 
a
  
a
  
 
  
 
Selection rule: v = ±1
Overtones: v = ±2, ±3, …
Must also have a change in polarizability
Classical Description does not suggest any difference
between Stokes and Anti-Stokes intensities
1
0
vibh
kT
N
e
N



16

17. The Raman polarization
The Raman Polarization is a property of waves that can oscillate with more
than one orientation EMR or waves, such as light and gravitational wave
exhibit polarization.
Polarization state :- the shape traced out in a fixed plane by the electric
vector as such a plane wave passes over it is a description of the
polarization.
E.g. linear polarization
circular polarization
elliptical polarization
orthogonally polarization
Polarization changes are necessary to form the virtual state and hence the
Raman effect.
17

18. Condition for raman spectroscopy
Vibrational modes that are more polarizable are more Raman-active
Examples:
– N2 (dinitrogen) symmetric stretch
cause no change in dipole (IR-inactive)
cause a change in the polarizability of the bond – as the bond gets
longer it is more easily deformed (Raman -active)
– CO2 asymmetric stretch
cause a change in dipole (IR-active)
Polarizability change of one C=O bond lengthening is cancelled by
the shortening of the other – no net polarizability (Raman-inactive)
Some modes may be both IR and Raman-active, others
may be one or the other!
18

19. Condition for raman spectroscopy
 Raman spectra occurs as a result of oscillation of a dipole
moment, induced in a molecules by the oscillating electric field of
an incident wave.
 As the induced dipole moment is directly proportional to the
polarisability of the molecules, the molecules must possess
anisotropic polarisability which should change during molecular
rotation or vibration for vibrational or rotational-vibrational raman
spectra.
 Anisotropic polarisability depends upon the orientation of the
molecules.
 In the presence of an electric field, the electron cloud of an atom
or molecules is distorted or polarised.
19

20. Mutual Exclusion Principle
For molecules with a center of symmetry, no IR active
transitions are Raman active and vice versa
Symmetric molecules
IR-active vibrations are not Raman-active.
Raman-active vibrations are not IR-active.
O = C = O O = C = O
Raman active Raman inactive
IR inactive IR active
20

21. Raman
Instrumentation
21

22. There are following component
involves.
1. Laser or source of light
2. Filter
3. Sample holder
4. detector
22

23. The block design dispersive Raman scattering
system:
Radiation
sources
Sample
Wavelength
selector
Detector
InGaAs or
Ge
RecorderDetector
InGaAs or
Ge
Recorder
Detector
InGaAs or
Ge
Recorder
Block diagram
23
90·

24. Flow diagram dispersive Raman scattering system:
24

25. Schematic diagram dispersive raman scattering
system
25

26. 1. Laser or source of light
• Lasers are generally the only source strong enough to scatter lots of light and
lead to detectable raman scattering.
• Lasers operate using the principle of stimulated emission.
• Electronic population inversion is required to achieve gain via stimulated
emission (before the fluorescence lifetime is reached)
• Population inversion is achieved by “pumping” using lots of photons in a
variety of laser gain media
26

27. List of Various laser source
S.No. Laser wavelength
01 Nd:YAG 1064nm
02 He:Ne 633nm
03 Argon ion 488nm
04 GaAlAs diode 785nm
05 Co2 10600nm
06 Ti-Sapphire 800nm
27

28. A :- He:Ne laser
• Filled with 7:1 He & Ne gas optimum output of 6328 Å
• High voltage excitation is preferred
B :- Nd:YAG System
• A typical laser system –the neodymium-doped yttrium aluminum garnet or Nd+3
• YAG is a cubic crystalline material
• Crystal field splitting causes electronic energy level splitting
• Nd:YAG laser are optically pumped using a flash tube or laser diodes.
• These are the one of the most common type of laser.
• It emits 1064 nm wavelength
28

29. 2.Filter
• It is therefore essential to have monochromatic radiations.
• 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 ferricyanide or iodine in CCl2 may be used as a
monochromator.
29

30. 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 generally bigger in size than those for liquids.
• Solids are 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 inorganic
compounds in raman spectroscopy. 30

31. 4.detector
• Researchers traditionally used single points detectors such as
photocounting, photomultiplier(PMT), not because of the weakness of a
typical raman signal, longer exposure times were often required to obtains
raman spectrum of a decent quality.
• Now days multichannel detectors like photodiode arrays(PDA), charged
couple devices(CCD)
• Sensitivity & performance of modern CCD detectors are high.
31

32. APPLICATION
Pharmaceuticals and Cosmetics:-
• Compound distribution in tablets
• Blend uniformity
• High throughput screening
• API concentration
• Powder content and purity
• Raw material verification
• Polymorphic forms
• Crystallinity
• Contaminant identification
• Combinatorial chemistry
• In vivo analysis and skin depth profiling
32

33. • Geology and Mineralogy
• Raman spectra of (top to bottom) olivine, apatite, garnet and
gypsum illustrating how Raman can be used for fast mineral ID.
• Gemstone and mineral identification
• Fluid inclusions
• Mineral and phase distribution in rock sections
• Phase transitions
• Mineral behavior under extreme conditions
33

34. Carbon Materialss
• Peak fitting of the D and G bands in a DLC spectrum
• Single walled carbon nanotubes (SWCNTs)
• Purity of carbon nanotubes (CNTs)
• Electrical properties of carbon nanotubes (CNTs)
• sp2 and sp3 structure in carbon materials
• Hard disk drives
• Diamond like carbon (DLC) coating properties
• Defect/disorder analysis in carbon materials
• Diamond quality and provenance
34

35. Semiconductors
• Photoluminescence image of a 3” MQW semiconductor
wafer, showing variation of emission peak width
• Characterisation of intrinsic stress/strain
• Purity
• Alloy composition
• Contamination identification
• Superlattice structure
• Defect analysis
• Hetero-structures
• Doping effects
• Photoluminescence micro-analysis
35

36. Life Sciences
• Multivariate clustering of spectra acquired from three bacterial
species, illustrating how Raman can be used to characterise and
distinguish bacteria at the single cell level.
• Bio-compatibility
• DNA/RNA analysis
• Drug/cell interactions
• Photodynamic therapy (PDT)
• Metabolic accretions
• Disease diagnosis
• Single cell analysis
• Cell sorting
• Characterisation of bio-molecules
• Bone structure
36

37. Differences between IR and Raman methods
S.No Raman IR
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 is Raman active if it
causes a change in polarisability.
Vibration is IR active if there is
change in dipole moment.
03 The molecule need not possess a
permanent dipole moment.
The vibration concerned should have a
change in dipole moment due to that
vibration.
04 Water can be used as a solvent. Water cannot be used due to its
intense absorption of IR.
05 Sample preparation is not very
elaborate, it can be in any state.
Sample preparation is elaborate
Gaseous samples can rarely be
used.
06 Gives an indication of covalent
character in the molecule.
Gives an indication of ionic
character in the molecule.
07 Cost of instrumentation is very
high
Comparatively inexpensive.
37

38. Advantages of Raman over IR
• Water can be used as solvent.
• Very suitable for biological samples in native state (because water
can be used as solvent).
• Although Raman spectra result from molecular vibrations at IR
• frequencies, spectrum is obtained using visible light or NIR
• radiation.
• =>Glass and quartz lenses, cells, and optical fibers can be used.
• Standard detectors can be used.
• Few intense overtones and combination bands => few spectral
overlaps.
• Totally symmetric vibrations are observable.
• Raman intensities a to concentration and laser power.
38

39. Advantages of IR over Raman
• Simpler and cheaper instrumentation.
• Less instrument dependent than Raman spectra because IR spectra
are based on measurement of intensity ratio.
• Lower detection limit than (normal) Raman.
• Background fluorescence can overwhelm Raman.
• More suitable for vibrations of bonds with very low polarizability
(e.g. C–F).
39

40. Several variations of Raman
spectroscopy
1. Surface-enhanced Raman spectroscopy (SERS) – Normally
done in a silver or gold colloid or a substrate containing silver or
gold. Surface plasmons of silver and gold are excited by the laser,
resulting in an increase in the electric fields surrounding the metal.
• Given that Raman intensities are proportional to the electric field,
there is large increase in the measured signal (by up to 1011).
• This effect was originally observed by Martin Fleischmann but the
prevailing explanation was proposed by Van Duyne in 1977.
• A comprehensive theory of the effect was given by Lombardi and
Birke.
40

41. 2. Resonance Raman spectroscopy
The excitation wavelength is matched to an
electronic transition of the molecule or crystal, so
that vibrational modes associated with the excited
electronic state are greatly enhanced. This is useful
for studying large molecules such as polypeptides,
which might show hundreds of bands in
"conventional" Raman spectra. It is also useful for
associating normal modes with their observed
frequency shifts.
41

42. 3. Surface-enhanced resonance Raman
spectroscopy (SERRS) – A combination of SERS
and resonance Raman spectroscopy that uses proximity to
a surface to increase Raman intensity, and excitation
wavelength matched to the maximum absorbance of the
molecule being analysed.
4. Coherent anti-Stokes Raman
spectroscopy (CARS) –
Two laser beams are used to generate a coherent anti-Stokes
frequency beam, which can be enhanced by resonance.
42

43. 5. Raman optical activity (ROA) – Measures
vibrational optical activity by means of a small difference in the
intensity of Raman scattering from chiral molecules in right-
and left-circularly polarized incident light or, equivalently, a
small circularly polarized component in the scattered light.
6. Spatially offset Raman spectroscopy (SORS)
7. Spontaneous Raman spectroscopy (SRS)
8. Optical tweezers Raman spectroscopy (OTRS)
43

44. 9. Angle-resolved Raman spectroscopy
10. Inverse Raman spectroscopy.
11. Tip-enhanced Raman
spectroscopy (TERS)
12. Surface plasmon polariton enhanced
Raman scattering (SPPERS)
13. Stand-off Remote Raman –
14. Fourier-transform Rama spectroscopyapplied
to photobiological systems.
44

45. 45