CHEM 333
RAMAN
SPECTROSCOPY
Rabia Aslam Chaudary
2012-10-0011
OUTLINE:
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Introduction
Explanation of Vibrational Spectrum
Quantum explanation
Examples
Applications
Advantages and Disadvantages
Future prospects
INTRODUCTION:
Raman spectroscopy is a valuable tool for the characterization of
materials due to its extreme sensitivity to the molecular environment
of the species of interest.
Information on molecular vibrations can provide much structural,
orientational, and chemical information that can assist in defining the
environment of the molecule of interest to a high degree of specificity.
The Raman effect is named after C.V. Raman, who, with K.S.
Krishnan, first observed this phenomenon in 1928. It belongs to the
class of molecular-scattering phenomena.
Energy
Range (eV)
λ
Electronic
8.28 – 0.828
150 – 1500
nm
Vibrational 0.828 –
0.0124
Rotational
1.24 X 10-2 –
1.24 X 10-4
1.5 – 100
µm
100 –
10,000 µm
Light scattering effects:
• A radiation which consists of photons undergoes undergo two types of
collisions with the molecules of a medium.
• Elastic Collisions: in which scattered photon has same energy as
incident photon. (Rayleigh scattering)
• Inelastic Collisions: in which scattered photon has more (Anti-stokes
Raman scattering) or less energy(Stokes Raman scattering) as
incident photon.
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1. Raman line (band)
A line (band) that is part of a Raman spectrum and corresponds to a characteristic
vibrational frequency of the molecule being probed.
2. Raman shift
The displacement in wave number of a Raman line (band) from the wave number of the
incident monochromatic beam. Raman shifts are usually expressed in units of cm-1.
They correspond to differences between molecular vibrational, rotational, or electronic
energy levels.
3. Raman spectroscopy
Analysis of the intensity of Raman scattering of monochromatic light as a function of
frequency of the scattered light.
4. Raman spectrum
The spectrum of the modified frequencies resulting from inelastic scattering when
matter is irradiated by a monochromatic beam of radiant energy. Raman spectra
normally consist of lines (bands) at frequencies higher and lower than that of the
incident monochromatic beam.
•Anti-Stokes Raman line: A Raman line that has a frequency higher than that of
the incident monochromatic radiation
•Stokes Raman line : A Raman line that has a frequency lower than that of the
incident monochromatic beam.
Because anti-Stokes scattering can occur only for molecules that are in an
excited vibrational or rotational state before scattering, the intensity of antiStokes radiation is significantly less than that of Stokes radiation at room
temperature.
Therefore, Raman spectroscopy generally uses Stokes radiation. Overall,
however, the total amount of inelastically scattered Stokes and anti-Stokes
radiation is small compared to the elastically scattered Rayleigh radiation.
Quantum mechanics Explanation:
A molecule in an electromagnetic field is distorted by the electrons to the positive
pole of the electric field and attraction of nuclei to the negative pole of the field.
The extent of distortion is called polarizibility.
Separation of charges produce a dipole and dipole moment per unit volume is
known as polarization.
The magnitude of polarization is given by:
The oscillating Electric field is and consequently polarization would be:
Now, an oscillating dipole moment is a source of radiation.
If molecules undergo internal motion like vibration or rotation then polarizibility
changes periodically.
This equation predicts three components of scattered radiation!
APPLICATIONS:
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Use of laser as an intense monochromatic light that overcomes weak Raman
scattering and exploits the advantages of Raman spectroscopy in the
vibrational analysis of surfaces and surface species overcoming the limitations
of infrared spectroscopy.
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Raman spectroscopy is accessible to the low-frequency region of the spectrum
thus giving a complete vibrational analysis of surface species.
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It is also valuable in probing surface processes in aqueous environments due
to extreme weakness of Raman scattering in water. This has made study of
corrosion possible where Raman scattering being weak in water does not
interfere with detection of metal corrosion products.
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In the study of metal oxides systems, Raman overcomes the problem with
infrared analysis which is the interference from absorption of the radiation
by the underlying bulk material. These species are strong to infrared but
only weak to moderate Raman scatters. Metal oxides exhibit strong
absorption in the infrared region.
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Raman spectroscopy has been extensively used to characterize the surface
structure of supported metal oxides. Example includes Raman investigation
of chemical species formed during calcination and activation of tungsten
trioxide catalysts supported on Silica and Alumina. Results show that
crystalline and polymeric forms of Tungsten trioxide are present on Silicon
dioxide supported surfaces.
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Another example would be of pyridine. Its ring-breathing vibrational modes are
extremely sensitive to the chemical environment.
The various interactions produce substantial shifts in the peak frequency of
symmetrical ring-breathing vibration of pyridine. In general, the more strongly
interacting the lone pair of electrons on the pyridine nitrogen, the larger the
shift to higher frequencies.
MODEL
COMPOUND
V1/cm
Nature of
interaction
Pyr
991
Neat Liquid
Pyr in CHCl3
998
H-bond
Pyr in CH2Cl2
992
No interaction
Pyr in CCl4
991
No interaction
Pyr in H2O
1003
H-bond
Pyr:ZnCl2
1025
Coordinately bound
Pyr N-Oxide
1016
Coordinately bound
Pyr: GaCl3
1021
Coordinately bound
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The weak Raman scattering of water makes the Raman analysis of polymers
in aqueous media particularly attractive. \One of the major advantages of
Raman spectroscopy is the availability of the entire vibrational spectrum
using one instrument.
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The dependence of Raman scattering intensity on changes in polarizability
of molecule makes it particularly sensitive to symmetrical vibrations and to
vibrations involving larger atoms. In particular, Raman intensities are more
sensitive than infrared for the detection of C=C, C C, phenyl, C-S, and S-S
vibrations various types of degradation and polymerization have involved
observing changes in vibrational features that affect only a small part of the
polymer.
Raman spectroscopy has been used extensively to characterize the extent
of surface structural disorder in graphites. Graphites is experimentally
complicated by strong absorption of laser radiation, which damages the
surface significantly. To avoid surface decomposition during analysis, low
laser powers on a stationary graphite sample are used.
Raman microprobe analysis has been used with transmission electron
microscopy (TEM) to characterize vapor deposition of carbon films on alkali
halide cleavage.
Raman spectroscopy has also provided evidence for the intercalation of
Br2, ICl, and IBr as molecular entities.
Applications in Medicine:
Now-a-days Raman spectroscopic technology is utilized for the detection of
changes occurring at the molecular level during the pathological
transformation of the tissue. The potential of its use in urology is still in its
infancy and increasing utility of this technology will transform noninvasive
tissue diagnosis.
It has shown encouraging results in diagnosis and grading of the bladder
and the prostate. Raman microprobes have been used for the
characterization and identification of renal lithiasis.
ADVANTAGES
DISADVANTAGES
•Can be used with solids and liquids
•Can not be used for metals or alloys.
•No sample preparation needed
•The Raman effect is very weak. The
•Not interfered by water
detection needs a sensitive and highly
•Non-destructive
optimized instrumentation
•Highly specific like a chemical
•Fluorescence of impurities or of the
fingerprint of a material
sample itself can hide the Raman
•Raman spectra are acquired quickly
spectrum
within seconds
•Sample heating through the intense
•Samples can be analyzed through
laser radiation can destroy the sample
glass or a polymer packaging
or cover the Raman spectrum
•Laser light and Raman scattered light
can be transmitted by optical fibers
over long distances for remote
analysis
•Raman spectra can be collected from
a very small volume (< 1 µm in
diameter)
•Inorganic materials are normally
easily analyzed by Raman than by
infrared spectroscopy
FUTURE OF RAMAN SPECTROSCOPY:
The future would see the development of optical fiber probes to incorporate
them into catheters, endoscopes, and laparoscopes that will enable the
urologist to obtain information during the operation.
Raman spectroscopy is an emerging technique that is able to interrogate
biological tissues, that gives us an understanding of the changes in molecular
structure that are associated with disease development. The potential
applications of Raman spectroscopy may herald a new future in the
management of various malignant, premalignant, and other benign conditions
in urology.
REFERENCES:
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11.
C.V. Raman and K.S. Krishnan, Nature, Vol 122, 1928, p 501
D.A. Long, Raman Spectroscopy, McGraw-Hill, 1977
M.C. Tobin, Laser Raman Spectroscopy, John Wiley & Sons, 1971
D.C. O' Shea, W.R. Callen, and W.T. Rhodes, Introduction to Lasers and Their
Applications,
Addison-Wesley, 1977
Spex Industries, Metuchen, NJ, 1981
E.E. Wahlstrom, Optical Crystallography, 4th ed., John Wiley & Sons, 1969
M. Delhaye and P. Dhemalincourt, J. Raman Spectrosc., Vol 3, 1975, p 33
P. Dhamelincourt, F. Wallart, M. Leclercq, A.T. N'Guyen, and D.O. Landon, Anal. Chem.,
Vol 51, 1979,p 414A
P.J. Hendra and E.J. Loader, Trans. Faraday Soc., Vol 67, 1971, p 828
E. Buechler and J. Turkevich, J. Phys. Chem., Vol 76, 1977, p 2325
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