Raman Spectroscopy

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• Raman Spectroscopy
• Raman effect is a 2-photon scattering process
• These processes are inelastic scattering
• Stokes scattering energy lost by photon
• ? ? (( ? ? )) ?
• Photon in Photon out
• No vibration Vibration
• Anti-Stokes scattering energy gained by photon
• ?(( ? ? )) ? ? ?
• Photon in Photon out
• Vibration No vibration

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• But dominant process is elastic scattering
• Rayleigh scattering
• ? ? ? ? ?
• Photon in Photon out
• No vibration No vibration
• If incident photon energy E vibration energy v,
  then
• in terms of energy, photon out has energy
• E-v Stokes scattering
• Ev anti-Stokes scattering
• E Rayleigh scattering

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Representation in terms of energy levels
Arrow up laser photon in Arrow down Raman
scattering out
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Explanation of Raman effect click under
scattering http//gepasi.dbs.aber.ac.uk/roy/ftir/
emspec.htm Here is another explanation, and a
Raman spectrometer http//www-personal.umich.edu/
jshaver/virtual/labeled/desc.html Or http//www-p
ersonal.umich.edu80/jshaver/virtual/explain.html

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Typical Raman spectrum Plot of signal intensity
vs Raman shift (Raman shift, in cm-1 energy of
photon in-energy of photon out)
shows 3 vibrations of octahedral BiCl63-
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Selection rule for Raman spectrum Vibration is
active if it has a change in polarizability,
?. Polarizability is the ease of distortion of a
bond. For Raman-active vibrations, the incident
radiation does not cause a change in the dipole
moment of the molecule, but instead a change in
polarizability. In starting the vibration going,
the electric field of the radiation at time t, E,
induces a separation of charge (i.e. between the
nuclear protons and the bonding electrons). This
is called the induced dipole moment, P. (Dont
confuse it with the molecules dipole moment, or
change in dipole moment, because this is often
zero). P ?E
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• Example There are 4 normal modes of CO2. Only ?1
  is Raman active
• is dipole moment
• is polarizability
• Q is vibration coordinate, The slopes
• are measured at Q 0 (I.e. at the equilibrium
  position).

Change in dipole moment, ?, and polarizability,
?, during CO2 vibrations
?1 ?3
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Uses of Raman Spectroscopy Raman spectroscopy has
become more widely used since the advent of
FT-Raman systems and remote optical fibre
sampling. Previous difficulties with laser
safety, stability and precision have largely been
overcome. Basically, Raman spectroscopy is
complementary to IR spectroscopy, but the
sampling is more convenient, since glass
containers may be used and solids do not have to
be mulled or pressed into discs.
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Applications of Raman spectroscopy Qualitative
tool for identifying molecules from their
vibrations, especially in conjunction with
infrared spectrometry. Quantitative Raman
measurements a) not sensitive since Raman
scattering is weak. But resonance Raman spectra
offer higher sensitivity, e.g. fabric dyes
studied at 30-50 ppb. b) beset by difficulties
in measuring relative intensities of bands from
different samples, due to sample alignment,
collection efficiency, laser power. Overcome by
using internal standard.
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Raman and fluorescence spectra The diagram shows
some of the energy levels of the uranyl ion.
UO22    
20000 cm-1 ? Energy    
___________ 800
cm-1 ___________
0 cm-1 The vibrational level at 800 cm-1
is the totally symmetric stretch. The electronic
levels are fairly continuous above 20000
cm-1. What happens if you excite with a laser at
(a) 15000 cm-1? (b) 21000 cm-1? (c) 22000 cm-1?
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Raman vs IR spectroscopy  RAMAN

IR Sample preparation usually simpler Liquid/
Solid samples must be free from dust Biological
materials usually fluoresce, masking
scattering Spectral measurements on vibrations
Halide optics must be used- made in the
visible region-glass cells expensive,
easily broken, may be used
water
soluble Depolarization studies are easily made
IR spectrometers not usually (laser radiation
almost totally linearly equipped with
polarizers polarized)
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More about polarizability ?, polarizability of
molecule, related to mobility of electrons (under
applied radiation field in our present case). For
atoms, same distortion is obtained for field in
any direction. Polarizability is Isotropic For
many molecules, polarizability depends on
direction of applied field, e.g. HH easier to
distort along bond than ? bond. Polarizability is
anisotropic   Variation of ? with direction is
described by polarizability tensor.  
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Calculation of Stokes and anti-Stokes intensity
ratio
The Raman spectrum was taken at 300 K using 1064
nm Nd-YAG radiation. Check the intensity ratio
of the ?1 features at ?278.5 cm-1. What can you
say about the intensity ratio of the band at 112
cm-1?
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Instrumentation Dispersive Raman
instruments Laser ?Sample ?Double or triple
monochromator
?
signal processing and output ?
Photomultiplier tube The monochromators are
required to separate the weak Raman signal from
the intense, nearby Rayleigh scattering. Typical
lasers are Ar (e.g. green line, 514.5 nm) or Kr
(e.g. yellow line, 530.9 nm).
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New Raman systemshttp//www.s-and-i.de/
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Fourier transform Raman spectrometer The Raman
instrument can be on the same bench as the FTIR.
Often, a YAGNd3 laser (1064 nm) is used to
excite the sample, so that the excitation energy
is lower than the absorption band energies of
organic systems. Fluorescence is then
minimized. Instruments may be combined with a
microscope, or optical fibre, so that scanning
over a few (microns)2 of surface area, and Raman
mapping is easily performed.
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Sampling techniques for Raman spectroscopy If the
sample is colourless, it does not absorb a
visible laser Raman spectroscopy is applicable to
solids, liquids or gases. Gases use gas cell
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Liquids and solids can be sealed in a glass
capillary
If the compound is colored, it can absorb the
laser, get hot and decompose. Some techniques
are Reduce the laser power (defocus) and/or
change wavelength Dilute the sample into a KBr
pellet Cool the sample Rotate or oscillate
the laser beam on the sample
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Number of bands in a Raman spectrum As for an IR
spectrum, the number of bands in the Raman
spectrum for an N-atom non-linear molecule is
seldom 3N-6, because polarizability change is
zero or small for some vibrations bands
overlap combination or overtone bands are
present Fermi resonances occur some vibrations
are highly degenerate etc
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High resolution Raman spectra can show splittings
due to isotopic mass effects, for example the ?1
Raman band of CCl4 (corresponding tp the totally
symmetric stretching vibration) is split into 5
components. 461.5 cm-1 is due to
35Cl4C 458.4 cm-1 is due to 35Cl337ClC 455.1
cm-1 is due to 35Cl237Cl2C What are the two
question marks? Why are these bands weak?
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Depolarization ratio of a vibrational mode in the
Raman spectrum may give information about the
symmetry of a vibration. ?p depolarization
ratio for polarized light Iy/Iz I?/I This
is different from the depolarization ratio for
unpolarized light, see Infrared and Raman
SpectraPart A., K. Nakamoto 5th Ed. Wiley 1997.
Pp. 97-101. 0? ?p lt0.75 Raman line is polarized
(p). Vibration is totally symmetric ?p 0.75.
Raman line is depolarized (dp). Vibration is not
totally symmetric.
CCl4
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Special types of Raman spectroscopy (see Hollas,
Modern Spectroscopy, Wiley) Resonance Raman (RR)
scattering When the laser excitation frequency is
near (or coincident) with an electronic
absorption band, intensity enhancement can occur
by a factor of 102-106, compared with normal
Raman scattering. Electronic transitions are
often localized in one part of a molecule, so
that RR provides information about vibrations of
the chromophore, especially those exhibiting a
large change in geometry between the two
electronic states. RR is used in analytical
chemistry to achieve detection limits 10-6-10-8
M. Surface enhanced Raman spectroscopy
(SERS) Raman scattering is enhanced (typically
by 103-106 times) when the analyte is adsorbed on
colloidal metallic surfaces, e.g. on colloidal
Ag prepared by reduction of Ag with citrate, in
particle size range 25-500 nm.
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Stimulated Raman scattering (SRS) The Raman
scattering from a laser is observed in the
forward direction from the sample (i.e. in the
same direction, or at a small angle to the
incident laser radiation). Vibrational
progressions are observed for certain
modes. Coherent anti-Stokes Raman spectroscopy
(CARS) Radiation from two lasers is incident on
the sample, and the intensity of the outgoing
wave energy gives information about the
vibrational modes of the sample. Hyper-Raman
spectroscopy Very weak scattering at twice the
laser frequency, 2?0, is called Hyper-Rayleigh
scattering. Similarly, Stokes and anti-Stokes
hyper-Raman scattering occur at 2?0??vib, where
?vib is a vibration frequency. The selection
rules differ from those of Raman scattering.
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Electronic Raman scattering Raman scattering can
occur from electronic states, as well as from
vibrations. At room temperature the bands are
very broad, and merge into the background. At low
temperatures (lt80 K), the bands sharpen, and give
information about the energies of electronic
states of the molecule.
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Books on Raman spectroscopy There are many in
CityU library, such as FT Raman spectroscopy, P.
Hendra et al., Ellis Horwood. Raman and IR
spectroscopy in biology and chemistry, J.
Twardowski and P. Anzenbacher, Ellis
Horwood. Also, there are some good short
chapters Ch 18 in Skoog, Holler, Nieman,
Principles of Instrumental Analysis, Saunders.
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Useful websites for Raman spectrometry Great site
covering all aspects http//www.spectroscopynow.co
m/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.h
tml Introduction to Raman spectroscopy http//www-
wilson.ucsd.edu/education/pchem/spectroscopy/sptyp
eraman.html Analysis of pesticides and
pharmaceuticals http//www.aua.gr/georgiou/ From
Encyclopaedia Brittanica Raman http//search.eb.c
om/bol/search?typetopicqueryOptical,infrared,
andRamanspectroscopyDbaseArticlesI3.x44I3.y
2 Andor Technology good site comparing Raman
and ir FT-Raman and applications http//www.andor
-tech.com/ then click on Contents, then Raman
on-line tour Peter Griffiths website about
FT http//www.ftir.chem.uidaho.edu/ Raman
analysis of olive oils (get the English page) by
Adrian Shaw http//pcjagg.dbs.aber.ac.uk/index.htm
l Study of molecular structures in supersaturated
solutions http//ihome.ust.hk/keckchan/spectrosco
py.html
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Useful websites for Raman spectrometry
Instrument manufacturers Thermo http//www.spectro
scopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-n
ews_detail-0-3606,00.html Renishaw instruments,
do a search at http//www.renishaw.com/client/star
t/index.asp Bruker instruments  http//www.optics
.bruker.com/  Nicolet instruments
http//www.nicolet.com/ and http//www.nicolet.com
/labsys/products/Raman_theory.html Jobin-Yvon
instruments website  http//www.isa-gs.co.uk/ Sci
media company about CCD camera
detector http//www.scimedia.com/english/camera/in
dex.htm ukap www.anamap.co.uk