Raman..................................ppt

Talk about raman

3. Polarizability is
the ability to form instantaneous 
dipoles.
It is a property of
matter.
Polarizabilities determine the dynamical response of a bound system to
external
fields, and provide insight into a molecule's internal structure.

The
polarizability 

α
 in 
isotropic
media is defined as the ratio of the induced 
dipole

moment
p of an atom to the electric field  E  that produces this dipole moment.


p=

αE

4. Raman Spectroscopy
•When
radiation passes through a transparent medium, the species present scatter a
fraction
of the beam in all directions.
•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 and furthermore that the shifts in wavelength depend upon the
chemical
structure of the molecules responsible for the scattering.
•The
theory of Raman scattering shows that the phenomenon results from the same
type
of quantized vibrational changes that are associated with infrared absorption.
Thus,
the difference in wavelength between the incident and scattered visible
radiation
corresponds to wavelengths in the mid-infrared region.
•The
Raman scattering spectrum and infrared absorption spectrum for a given species
often
resemble one another quite closely.
•An
important advantage of Raman spectra over infrared lies in the fact that water does
not
cause interference; indeed, Raman spectra can be obtained from aqueous
solutions.

•In
addition, glass or quartz cells can be employed, thus avoiding the inconvenience of
working
with sodium chloride or other atmospherically unstable window materials.

5. Raman spectroscopy is a
vibrational spectroscopy technique used to study the
molecular
structure and composition of materials. It works by analyzing the inelastic
scattering
of light (typically from a laser) when it interacts with the sample. This
scattered
light provides a unique "fingerprint" that reveals information about the
vibrational
modes of the molecules within the material. 

8. The
Raman Spectrum of CCl
4
Observed in
“typical”
Raman
experiments

0
= 20492 cm
-1

0
= 488.0 nm
Anti-Stokes lines
(inelastic scattering)
-218
Raman shift cm
-1


0
= (
s
- 
0
)
-200
Stokes lines
(inelastic scattering)
-400400 200
218
314
-314
-459
459
0
Rayleigh line
(elastic scattering)

10. Raman-Active Vibrational Modes
Modes that are more polarizable are more Raman-active
Examples:
–N
2
(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)
–CO
2 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!

11. Raman vs IR
1.
For a given bond, the energy shifts observed in a Raman experiment should be identical to the
energies
of its infrared absorption bands, provided that the vibrational modes involved are active
toward
both infrared absorption and Raman scattering. The differences between a Raman spectrum
and
an infrared spectrum are not surprising.
Infrared
absorption requires that a vibrational mode of
the
molecule have a change in dipole moment or charge distribution associated with it.
2.In
contrast, scattering involves a momentary distortion of the electrons distributed around a bond
in
a molecule, followed by reemission of the radiation as the bond returns to its normal state. In its
distorted
form, the molecule is temporarily polarized; that is, it develops momentarily an induced
dipole
that disappears upon relaxation and reemission. The Raman activity of a given vibrational
mode
may differ markedly from its infrared activity.
3.The
intensity or power of a normal Raman peak depends in a complex way upon the polarizability
of
the molecule, the intensity of the source, and the concentration of the active group. The power
of
Raman emission increases with the fourth power of the frequency of the source; however, the
advantage
of this relationship can seldom be taken away because of the likelihood that ultraviolet
irradiation
will cause photodecomposition. Raman intensities are usually directly proportional to
the
concentration of the active species.

13. Sample Illumination System
Sample
handling for Raman spectroscopic measurements is simpler than for infrared spectroscopy
because
glass can be used for windows, lenses, and other optical components instead of the more
fragile
and atmospherically less stable crystalline halides. In addition, the laser source is easily
focused
on a small sample area and the emitted radiation efficiently focused on a slit.
Consequently,
very small samples can be investigated. A common sample holder for
nonabsorbing
liquid samples is an ordinary glass melting-point capillary.
•Liquid Samples:
A major advantage of sample handling in Raman spectroscopy compared with
infrared
arises because water is a weak Raman scatterer but a strong absorber of infrared radiation.
Thus,
aqueous solutions can be studied by Raman spectroscopy but not by infrared. This
advantage
is particularly important for biological and inorganic systems and in studies dealing
with
water pollution problems.
•Solid Samples:
Raman spectra of solid samples are often acquired by filling a small cavity with
the
sample after it has been ground to a fine powder. Polymers can usually be examined directly
with
no sample pretreatment.

14. Raman Spectrometers


Raman
spectrometers were similar in design and used the same type of components as the classical
ultraviolet/visible
dispersing instruments. Most employed double grating systems to minimize the spurious
radiation
reaching the transducer. Photomultipliers served as transducers. Now Raman spectrometers being
marketed
are either Fourier transform instruments equipped with cooled germanium transducers or multichannel
instruments
based upon charge-coupled devices.

16. APPLICATIONS OF RAMAN SPECTROSCOPY
Raman Spectra of Inorganic Species
The
Raman technique is often superior to infrared for spectroscopy investigating
inorganic
systems because aqueous solutions can be employed.
In
addition, the vibrational
energies
of metal-ligand bonds are generally in the range of 100 to 700 cm
-1
,
a region of
the
infrared that is experimentally difficult to study.
These
vibrations are frequently
Raman
active, however, and peaks with

values in this range are readily observed.
Raman
studies are potentially useful sources of information concerning the composition,
structure,
and stability of coordination compounds.
Raman Spectra of Organic Species
Raman
spectra are similar to infrared spectra in that they have regions that are useful for
functional
group detection and fingerprint regions that permit the identification of specific
compounds.
Raman spectra yield more information about certain types of organic
compounds
than do their infrared counterparts.
Biological Applications of Raman Spectroscopy
Raman
spectroscopy has been applied widely for the study of biological systems. The
advantages
of his technique include the small sample requirement, the minimal sensitivity
toward
interference by water, the spectral detail, and the conformational and
environmental
sensitivity.
Raman spectra for catalysis research

17. Other
unique examples:
Resonance
Raman spectroscopy: strong enhancement (10
2

–
10
6

times)
of
Raman
lines by using an excitation frequency close to an electronic
transition
(Can detect umol or nmol of analytes).
Surface-enhanced
Raman (SERS): an enhancement obtained for samples
adsorbed
on colloidal metal particles.
Coherent
anti-Stokes Raman (CARS): a non-linear technique using two
lasers
to observe third-order Raman scattering – used for studies of
gaseous
systems like flames since it avoids both fluorescence and
luminescence
issues.

19. Comparison
of IR and Raman Spectroscopy
Advantages of Raman over IR:
–Avoids many interferences from solvents, cells and sample
preparation methods
–Better selectivity, peaks tend to be narrow
–Depolarization studies possible, enhanced effects in some cases
–Can detect IR-inactive vibrational modes
Advantages of IR over Raman:
–Raman can suffer from laser-induced fluorescence and
degradation
–Raman lines are weaker, the Rayleigh line is also present
–Raman instruments are generally more costly
–Spectra are spread over many um in the IR but are compressed
into several nm (20-50 nm) in the Raman
Final conclusion – they are complementary techniques!

20. Types of Raman Spectroscopy
At
least 25 variations of Raman spectroscopy have been developed. The usual purpose is
to
enhance the sensitivity (e.g., 
Surface-enhanced
Raman spectroscopy (SERS)
),
to
improve
the spatial resolution (Raman microscopy), or to acquire very specific
information
(resonance Raman).
1.Normal Raman spectroscopy
2.Resonance Raman spectroscopy
3.Angle-resolved Raman spectroscopy
4.Optical tweezers Raman spectroscopy (OTRS)
5.Spatially offset Raman spectroscopy (SORS)
6.Raman optical activity
 (ROA)
7.Transmission Raman spectroscopy (TRS)
Spontaneous (or far-field) Raman spectroscopy
Enhanced (or near-field) Raman spectroscopy
1.Surface-enhanced Raman spectroscopy
 (SERS)
2.Surface-enhanced resonance Raman spectroscopy (SERRS)
3.Tip-enhanced Raman spectroscopy
 (TERS)
4. Surface plasmon polariton
 enhanced Raman scattering (SPPERS)
Non-linear Raman spectroscopy
1.Hyper Raman spectroscopy,
2.Stimulated Raman spectroscopy (SRS)
3.Inverse Raman spectroscopy
and
4.Coherent anti-Stokes Raman spectroscopy
 (CARS)

21. Surface-Enhanced Raman Spectroscopy (SERS)
Surface-enhanced Raman spectroscopy or surface-enhanced Raman scattering (SERS)
is a
surface-sensitive
technique that enhances Raman scattering by molecules adsorbed on rough
metal surfaces or
by nanostructures such as plasmonic-magnetic silica nanotubes. The enhancement
factor
can be as much as 10
10
 to
10
11
, which
means the technique may detect single molecules.
SERS provides
detailed information about the molecular composition, structure, and environment of
the
analyzed molecules or analyte. Additionally, SERS can detect molecules at very low
concentrations,
often down to single molecule levels.

24. Applications of SERS
•Bioanalysis:
•SERS
is used to analyze biomolecules like DNA, RNA, proteins, and other cellular components, making it valuable
for
diagnostics and research in life sciences. It can also be used to detect cancer markers, bacteria, and viruses. 
•Medical Diagnosis and Treatment:
•SERS
can be used for label-free detection of various diseases, including cancer, infectious diseases, and
inflammatory
conditions. It can also be used to monitor drug efficacy and personalize treatments. 
•Environmental Monitoring:
•SERS
is employed for detecting pollutants, toxins, and other harmful substances in water, air, and soil. This includes
the
detection of pesticides, heavy metals, and other contaminants. 
•Food Safety:
•SERS
can be used to detect foodborne pathogens, toxins, and adulterants, ensuring the safety and quality of food
products. 
•Material Science:
•SERS
is used to study the properties and behavior of various materials, including nanomaterials and thin films. 
•Pharmaceuticals:
•SERS
is used in drug discovery, drug analysis, and drug delivery studies. It can help in understanding drug-target
interactions
and optimizing drug formulations. 
•Forensics:
•SERS
is used for trace evidence analysis, such as detecting gunshot residue, explosives, and other illicit substances. 
•Explosives Detection:
•SERS
is a valuable tool for detecting explosives and other hazardous materials due to its high sensitivity and ability
to
identify molecules even at very low concentrations. 
•Art Conservation and Cultural Heritage:
•SERS
can be used to analyze pigments, dyes, and other materials used in art and cultural artifacts, aiding in their
preservation
and restoration. 
•Point-of-Care Diagnostics:
•SERS-based
sensors are being developed for use in point-of-care diagnostics, enabling rapid and accurate testing in
settings
outside of traditional laboratories.