Raman spectroscopy- General Talak

Raman Spectroscopy
(Data Analysis)
Introduction, Symmetry & Point group,
Character table, Resonance Raman,
Instrumentation and Depth profiling
K. Kamalakkannan,
University of Madras
Kamalakkannan.k.123@gmail.com

Department of Nuclear Physics, University of Madras
Raman Spectroscopy- Short Term Training Program, Sathyabama University
Introduction
 When monochromatic radiation is incident upon a sample then this light will interact with the sample in some
fashion. It may be reflected, absorbed or scattered in some manner.
 We discussing about the scattering, the species present scatter a fraction of the beam in all directions, which
gives information about molecular structure
 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.

Introduction

Introduction
Raman is based on scattering. The sample is irradiated with a
coherent source, typically a laser. Most of the radiation is elastically
scattered (called the Rayleigh scatter).
A small portion is in elastically scattered (Raman scatter, composed
of Stokes and anti-Stokes portions). This latter portion is what we
are particularly interested in because it contains the information in
which we are interested.

Introduction
hν0
Incident light
E
Sample
(Monochromater)
E
R Anti StokesStokes
δE
δE = Transition between Vibrational Energy level
Electrons in molecules oscillate with respect to electric field – Go to higher energy state and goes to higher
Energy level and return with different wavelength.
Change in polarizability- how amplitude the e-’s moves (transient) with respect to the oscillating e-’s field (given).

Introduction
Selection Rule for IR and Raman spectroscopy:
δE = ± 1
Electron polarization changes during vibration : Raman
-Ability to move the electrons
-Applied E- field
-The way of moves of electrons
The scattering molecule must be anistropically polarizable ( Different with different direction)
For x- direction- Electron move in y- direction.
Dipole changes : IR
extend compress

Symmetry and Character Table-
Introduction

Symmetry in Group theory
Symbols for operation
Identity E Do nothing
Mirror plane σ Reflection
Proper
Rotation
Cn
m
Rotation
Improper
Rotation
(This is a line)
Sn
m
Rotation &
Reflection
n= fraction of circle to Rotate
m= how many times repeat the action
C2
=
360/2= 180
Similarity or exact correspondence between different
things.

Symmetry labels and character Table
Functions Quadratic function
(Sub shells in a group)
Characters: 1 is Symmetry
-1 is anti symmetry
Functions: X, Y, Z are IR (Translational) and
Rz, Ry, Rx = Rotation about axis
Quadratic function: Raman active

Mulliken symbols
A Symmetric with respect to the principle rotation axis
B Asymmetric with respect to the principle rotation axis
E Doubly degenerate
T Triply degenerate
For example:
6H-SiC, the accessible phonon vibration modes of high symmetry points are A1, E1 and E2 from the
irreducible representation of Γ = 6(A1 + B1 + E1 + E2) at the Brillouin zone with the space group symmetry
C6V .

Raman Spectroscopy: Classical Treatment
• Number of peaks related to degrees of freedom
DoF = 3N - 6 (bent) or 3N - 5 (linear) for N atoms
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
Raman + IR: 3657 cm–1
Raman + IR: 3756 cm–1
RamanRaman + IR: 1594 cm–1
H2O

Exclusion principle
If a molecule has an center of symmetry (inversion center),
none of its modes can be Raman and IR active.
But Raman or IR active (Not both).

Main Optical Transitions: Absorption, Scattering, and Fluorescence
Electronic
Ground State
1st Electronic
Excited State
ExcitationEnergy,σ(cm–1
)
Vib.
states
4,000
25,000
0
IR
2nd Electronic
Excited State
σ
σ σemit
fluorescence
Impurity
σemit
fluorescence
UV/Vis
Fluorescence
σemitσ
Elastic
Scattering
(Raleigh)

Raman Spectroscopy: Absorption, Scattering, and Fluorescence
Electronic
Ground State
1st Electronic
Excited State
Vib.
states
4,000
25,000
0
IR
σ
σ σemit
2nd Electronic
Excited State
Raman
∆σ=σemit–σ
σ ±∆σ ∆σ
Resonance Raman
∆σ=σemit–σ
Stokes Anti-Stokes

Raman Spectroscopy: Absorption, Scattering, and Fluorescence
ExcitationEnergy,σ(cm–1
)
Vib.
states
4,000
25,000
0
fluorescence
IR
σ
σ σemit
Raman
∆σ=σemit-σ
σ ∆σ
fluorescence
Impurity
Fluorescence
= Trouble
Stokes Anti-Stokes

Photon-Molecule Interactions

Fluorescence Background in Raman Scattering

Resonance Raman scattering
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resonant Raman
v=1 v=1
v=0 v=0
Stokes Stokes
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virtual level virtual level
Vis Vis
514nm 632nm
Wavelength (nm)

Resonance Raman scattering

Raman vs Infrared Spectra

Difference between Raman & IR
RAMANAN
•It is due to the scattering of light by the vibrating molecules.
•The vibration is Raman active if it causes a
change in polarisability.
•The molecule need not possess a permanent dipole moment.
•Water can be used as a solvent.
•Sample preparation is not very elaborate, it
can be in any state.
•Gives an indication of covalent character in the molecule.
•Cost of instrumentation is very high
INFRA RED
•It is the result of absorption of light by
vibrating molecules.
•Vibration is IR active if there is change in
dipole moment.
•The vibration concerned should have a
change in dipole moment due to that
vibration.
•Water cannot be used due to its intense
absorption of IR.
•Sample preparation is elaborate
Gaseous samples can rarely be used.
•Gives an indication of ionic character in the
molecule.
•Comparatively inexpensive.

Spectra observations

Instrumentation - Basic design
Light source:
- generally a laser to get required intensity of light
for reasonable S/N
•Raman scattering is only 0.001% of light source
- Doesn’t have to be in IR region, since look at
changes around central peak.
•visible source used because of high intensity
•allows use of glass/quartz sample cells & optics
•UV/Vis type detectors (photomultiplier tubes)

Raman Instrumentation

Laser Source

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 - concentration and laser power.

Second order Raman scattering
All above is “first order” scattering, the excitation of only one
phonon.
It can be that two phonons can be excited, “second order”
scattering.
The wavelength selection rule in this case is that the two phonons
must have equal wave vectors (or wavelength) but must be
travelling in opposite directions.
Second order scattering is usually weak, but can be enhanced by
various things.

Raman spectra of nanocrystalline GaN
0 1000 2000 3000 4000
frequency, cm-1
a-GaN:O+
a-GaN:O
nx-GaN:H
nx-GaN
325 nm
OH
N
2
Si
O
order
GaN
2nd
3rd
order

Can be determine…..
 Structure of the crystal
 Carrier concentration by peak shift
 Depth profiling- Structure disorder in depth wise- Confocal Raman- Transparent samples
 Amorphous behavior- Peak intensity
 Defect quantification- Normalization
 Quantification of disorder- Effect of various parameter: Doping, Implantation, Radiation damage etc.,

Depth profiling
Optically transparent samples can be studied in the z-direction.
Therefore confocal Raman microscopy is an extremely useful technique,
because it allows nondestructive sample investigation in the third dimension
yielding subsurface information without the need of sample preparation such
as a microtome cut. By moving the laser focus in z-direction either a
complete 2D or even 3D depth profile of a structured sample.

Depth profiling
The confocal microscope combines focused illumination with spatially filtered
detection, to collect photons that originate from a diffraction-limited sample volume and
reject photons that originate from outside that region. This arrangement ensures that
only light coming from the focal plane entirely reaches the detector. This accounts
for the confocal Raman microprobe’s ability to discriminate between parts of the sample
that are not at the same depth, thus allowing optical sectioning.

Surface-Enhanced Raman Spectroscopy (SERS)
Surface-Enhanced Raman Spectroscopy (SERS)
Surface enhanced Raman spectroscopy involves obtaining Raman spectra in the
usual way on samples that are adsorbed on the surface of colloidal metal particles
(usually silver, gold, or copper) or on roughened surfaces of pieces of these metals. For
reasons that are not fully understood, the Raman lines of the adsorbed molecule are
often enhanced by a factor of 103
to 106
. When surface enhancement is combined with
the resonance enhancement technique discussed in the previous section, the net
increase in signal intensity is roughly the product of the intensity produced by each of
the techniques. Consequently, detection limits in the 10-9
to 10-12
M range have been
observed.

.
Probability of Emission Observed Intensity
Raleigh scattering >> Stokes >> anti-Stokes
difference in population of energy levels of vibrational transitions

Comparison of Raman and IR Spectra

Dispersion for InN
The vibrations are waves, labelled phonons, with wave vector
q =2 /wavelength (of the phonon)
That can range from -p/a to +p/a, a the lattice constant ≈ 0.5 nm. If
there are N ions in a unit cell then there are 3N different vibrational
modes, each with its own frequency, for each value of wave vector.
So we can draw a “dispersion curve”.

Dispersion for InN
The phonon “dispersion”, meaning the frequency as a function of wavelength, is usually
plotted as frequency vs (wavelength)-1, across the Brillouin zone. Then infinite
wavelength is at zero, the zone centre, commonly labeled the (gamma) point. It’s the
frequencies along the left-hand edge that have infinite wavelength.
In a perfect crystal only the zone centre phonons can be Raman active, and
usually not even all of those are.

Raman Spectroscopy
 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 –1930 – C.V. Raman wins Nobel Prize in PhysicsC.V. Raman wins Nobel Prize in Physics
 1961 – Invention of laser makes Raman experiments reasonable1961 – Invention of laser makes Raman experiments reasonable
 1977 – Surface-enhanced Raman scattering (SERS) is discovered1977 – Surface-enhanced Raman scattering (SERS) is discovered
 1997 – Single molecule SERS is possible1997 – Single molecule SERS is possible

Raman spectroscopy- General Talak

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