principle instrumentation working of the raman spectroscopy is explained

1. RAMAN
SPECTROSCOPY

2. HISTORY

3. Light scattered by molecules is shifted in frequency by diffrences in
the vibrational energy levels
-Stokes line
-Anti Stokes line
RAMAN EFFECT

4. What is Raman spectroscopy?
Raman Spectroscopy is a non-destructive chemical analysis technique which
provides detailed information about chemical structure, phase and polymorphy,
crystallinity and molecular interactions. It is based upon the interaction of light
with the chemical bonds within a material.
Raman is a light scattering technique, whereby a molecule scatters incident light
from a high intensity laser light source. Most of the scattered light is at the same
wavelength (or colour) as the laser source and does not provide useful
information – this is called Rayleigh Scatter. However a small amount of light
(typically 0.0000001%) is scattered at different wavelengths (orcolours),
which depend on the chemical structure of the analyte – this is called Raman
Scatter.

5. RAYLEIGH AND RAMAN
 Rayleigh scattering:
occurs when incident EM radiation induces an oscillating dipole in a
molecule, which is re-radiated at the same frequency
 Raman scattering:
occurs when monochromatic light is scattered by a molecule, and the
scattered light has been weakly modulated by the characteristic
frequencies of the molecule
– STOKES LINE
– ANTI-STOKES LINE

8. Stokes and anti-Stokes Raman spectral lines of CCl4

9. Incident radiation excites “virtual states” (distorted or polarized
states) that persists for a shorter time (10^-14 secs)
Inelastic scattering of a photon when it is incident on the
electrons in a molecule
STOKES LINE
When inelastically-scattered, the photon loses some of its
energy to the molecule (Stokes process). It can then be
experimentally detected as a lower-energy scattered photon
ANTI STOKES LINE
The photon can also gain energy from the molecule (anti-Stokes
process)

11. the “deformability” of a bond or a
molecule in response to an applied
electric field.
It is the most important selection
rule for Raman spectrum
Intensity of the Raman peak
depends on Polarisabilty of each
molecules of a substance

13. INDUCED ELECTRIC DIPOLE MOMENT
An electric field can distort the electron cloud
of a molecule, thereby creating an “induced”
electric dipole moment
The oscillating electric field associated with
EM radiation will therefore create an
oscillating induced electric dipole moment
which in turn will emit, i.e. scatter, EM
radiation

14. •A linear molecule of N atoms has (3 translational deg. freedom+
2 rotational ) 3N-5 normal modes of vibration
•Non-linear molecule: 3N-6 modes
•Normal modes:
Stretching motion between two bonded atoms
Bending motion between three atom connected by two bonds
Out-of-plane deformation modes
Vibration of molecules

15. VIBRATIONAL MODES
SYMMETRICAL ASYMMETRICAL WAGGING
TWISTING SCISSORING ROCKING

16.  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)
RAMAN ACTIVE VIBRATIONAL MODES

17. SOURCE
SELECTORS AND FILTERS
DETECTORS
RAMAN INSTRUMENTATION

20. Lasers
Laser wavelengths ranging from ultra-violet through visible to near
infra-red can be used .
Ultra-violet: 244 nm, 257 nm, 325 nm, 364 nm
Visible: 457 nm, 473 nm, 488 nm, 514 nm, 532 nm, 633 nm, 660 nm
Near infra-red: 785 nm, 830 nm, 980 nm, 1064 nm

21. LASERS
•Gas lasers
•Solid-state lasers
•Semiconductor lasers
•Other: Dye laser (tunable)
•Metal-vapor laser (deep UV)
•Ti: Sapphire (solid-state tunable)

23. SOLID STATE LASER

24. SAMPLING SYSTEMS

25. SAMPLING SYSTEMS

26. RAYLEIGH LINE REJECTION
Optical filters
these optical components are placed in the Raman beam path, and are
used to selectively block the laser line (Rayleigh scatter) whilst allowing
the Raman scattered light through to the spectrometer and
detector. Each laser wavelength requires an individual filter.
Edge. An edge filter is a long pass optical filter which absorbs all
wavelengths up to a certain point, and then transmits with high
efficiency all wavelengths above this point.
Holographic notch. A notch filter has a sharp, discrete absorption
which for Raman is chosen to coincide with a specific laser wavelength.

27. DETECTORS
Multichannel
- Charge Coupled Device (CCD)
-Diode Arrays
Single Channel
- Avalanche Photodiode (APD)
- Photomultiplier (PM)
- Single Diodes

28. A CCD (Charge Coupled Device) is a
silicon based multichannel array detector
of UV, visible and near-infra
light. They are used for Raman
spectroscopy because they are extremely
sensitive to light (and thus suitable for
analysis of the inherently weak Raman
signal), and allow multichannel operation
(which means that the entire Raman
spectrum can be detected in a single
acquisition).
CCD DETECTOR

30. PHOTO
MULTIPLIER

31. Other
forms of Raman
spectroscopy

32. Hand held Raman
spectroscopes
Handheld Raman
instruments are useful for the
identification of chemicals
Designed for safe for use in
manufacturing plant
environment, for military and
chemical weapons
applications, etc…

33. RAMAN
RESONANCE
SPECTROMETER
 UV lasers allow for better
Raman performance, because of
the 1/4 dependence of scattering,
but fluorescence is a problem
 With lasers in the 245-266 nm
region, the Raman spectrum can
be “fit” in the region above the
laser but below the normal
Stokes-shifted fluorescence
spectrum

34. SURFACE
ENHANCED
RAMAN
SPECTROSCOPY
 SERS is a form of Raman
spectroscopy that involves a molecule
adsorbed to the surface of a
nanostructured metal surface which can
support local surface plasmon resonance
(LSPR) excitations
 The Raman scattering intensity
depends on the product of the
polarizability of the molecule and the
intensity of the incident beam; the
LSPR amplifies the beam intensity
when the beam is in resonance with
plasmon energy levels – leads to signal
enhancements of >106

35. Selection rules
•A mode will be Raman active if it induces a change in the
polarizability (I) of the molecule.
•Dipole moment induced by the electric field E of a laser photon is
P = I E
Change in polarizability Change in the volume of electron
cloud
•Symmetric stretching modes will be (intensely) Raman active, IR
inactive

36. At various conditions
Heating/Cooling – typically suitable for temperatures in the
range -196oC to 600oC, or ambient to 1500oC, these stages can be
used for solids, powders and liquids.
Catalysis – a variant of the heating/cooling stages above, but
designed to have preheated gases forced through a catalyst
matrix. Suitable for temperatures up to 1000oC, and gas pressures
up to 5bar.
Tensile Stress – allows structural changes in a sample to be
monitored under tensile stress. Forces up to 200N can be used with
these stages.

37. Pressure – Diamond Anvil Cells (DAC) allow analysis at
pressures up to 50GPa, with elevated temperatures.
Humidity – control of sample temperature and humidity allows
analysis of solvent-adsorbate interactions, and the effect of humidity
on a sample’s structure.

38. Semi-conductors
Stress, contamination, super lattices structure and
defect investigations, hetero structures, doping effects.
Polymers
Polymorphs identification, blend morphology,
monomers and isomers analysis, crystallinity,
orientation, polymerisation .
Geology / Mineralogy/Gemmology
Fluid inclusions, gemstones, phase transitions,
mineral behaviour under extreme conditions,
mineral structures
FIELDS OF APPLICATIONS

39. Carbon compounds
DLC (Diamond Like Carbon), nanotubes,
Fullerenes characterisation, diamond, graphite,
intercalation compounds, film quality analysis,
hard-disk coatings analysis
Life Science
Bio-compatibility, DNA analysis, drug/cell
interaction, immuno globulins, nucleic acids,
chromosomes, oligosaccharides, cholesterol, lipids,
cancer, metabolic accretions, inclusion of
foreign materials and pathology.
CONTINUED

40. Forensics
Illicit drugs and narcotics, paints, pigments,
varnishes, fibres, explosives, inks, gems and other
geological specimens, gunshot residues.
Chemistry
Phase transitions, catalysts, corrosion, oxides,
electrochemistry, solid lubricants, silicon compounds,
surfactants, emulsions, aqueous chemistry, solvents
analysis
CONTINUED

41. •Fluorescence can often contaminate Raman spectra.
The use of a Near-IR 785 nm laser radically reduces the
possibility of fluorescent contamination, but there can
still be some fluorescent problems.
• More expensive
•Sample heating through the intense laser radiation can destroy the sample
or cover the Raman spectrum
•It is not suitable for metal alloys.
LIMITATIONS

42. Can be used with solids and liquids
It is highly non-destructive
No sample preparation needed
Not interfered by water
Non-destructive
Highly specific like a chemical fingerprint of a material
Raman spectra are acquired quickly within seconds
Samples can be analyzed through glass or a polymer packaging
Laser light and Raman scattered light can be transmitted by optical
fibers over long distances for remote analysis
ADVANTAGES