Lord Rutherford, 1930
April 10th, 2014
1. Introduction
What is Spectroscopy?
• Spectroscopy: The study of the interaction of
electromagnetic radiation(energy) with matter and can
be used to obtain information about it.
simple
complicated
Sir
Chandrasekhara
Venkata Raman
(1888-1970)
Who is Raman?
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In 1921 Raman took a tour to Europe as a
delegate of Universities’ congress
Raman was awarded the
Nobel Prize in Physics in
1930 for his discovery.
Awestruck by the grandeur of the Mediterranean
sea, its beauty and blueness, the more he saw,
the more did his wonder grow
Performed experiment on the ship by taking a
Nicole prism and observing at Brewster’s angle.
Demolished the theory that the blueness of sea is
the reflection of blue of the sky rather than from
scattering by the water.
In 1928, C. V. Raman discovers that small changes occur at
the frequency of a small portion of the light scattered by
molecules. The changes reflect the vibrational properties of
the molecule.
8th March, Note sent to Nature by Raman and Krishnan is
rejected by a referee, but published by the Editor
2. Theory
Principle of Raman Spectroscopy
Raman spectra are acquired by
irradiating a sample with a
powerful laser source of visible
or near-infrared monochromatic
radiation. During irradiation, the
spectrum of the scattered
radiation is measured at some
angle with a suitable
spectrometer. At the very most,
the intensities of Raman lines are
very small of the intensity of the
source; as a consequence, their
detection and measurement are
somewhat difficult.
Incident Laser
Scattered Light
Sample
Raleigh Scatter (same wavelength as incident light)
Raman Scatter (new wavelength)
Classical Mathematical Argument:

Consider the time variation of the dipole moment induced by incident
radiation (an EM field):
Induced dipole moment
 (t )   (t ) (t )
EM field
polarizability

*An electric field
applied to a
molecule results
in its distortion,
and the
distorted
molecule
acquires a
contribution to
its dipole
moment
If the incident radiation has frequency  and the polarizability of the molecule
changes between min and max at a frequency int as a result of this
rotation/vibration:
 (t )    12  cos intt  0 cos t
mean polarizability

 = max - min
Expanding this product yields:
 (t )  0 cos t  14 0 cos(  int )t  cos(  int )t 
Rayleigh line
Anti-Stokes line
Stokes line
Stokes/Anti-Stokes
Schematic diagram of the process:
Note that the transitions (scattering) take 10-14
seconds or less!
atom or a molecule
absorbs energy from the
incident photon and
jumps to a higher energy
state (called the virtual
level) for a while and
then emits the photon of
the same energy (hence
same wavelength too),
and goes back to its
original energy state.
ground state to virtual level and
may decide to sit on one of its
vibrational excited energy level by
emitting a light with lower energy
(hence higher wavelength). It called
Stokes lines.
vibration level to virtual level to
ground state of the atom/molecule .
It called anti-Stokes lines.
Stokes/Anti-Stokes Animation
• Energy transferred from incident
light to molecular vibrations
hn
Inelastic Scattering
h(n -+) n1)
hn
Energy
Virtual Level
3
2
1
0
Rayleigh
(elastic) Scattering
Raman (inelastic)
Scattering
S0
difference in energy
Raman Spectrum
Ideal Raman spectrum:
-Raman spectrum are plotted with
respect of the laser frequency such that
Rayleigh band lies at 0 cm-1
Rayleigh Scattering
-on this scale, band positions will lie at
frequency that correspond to energy
level of different functional group.
Intensity
Stokes Shift
Anti-Stokes Shift
300
200
100
0
-100
-200
-the Stokes lines are stronger because
the population of molecules at n=0 is
much larger than at
n=1 by the Maxwell-Boltzmann
distribution law.
-300
Raman Shift (cm-1)
Raman shifts are typically reported in wavenumbers, which have units of inverse length.
In order to convert between spectral wavelength and wavenumbers of shift in the
Raman spectrum, the following formula can be used:
λ0 = excitation wavelength,
λ1 = Raman spectrum wavelength
Raman Spectrum
Practical Raman spectrum:
-Raman spectra are usually presented as
just the Stokes spectra with the anti-Stokes
spectra omitted.
-The only inconsistent feature is in the way
in which the wavenumber scale is displayed,
sometimes from high to low wavenumber but
often from low to high wavenumber.
Molecular Vibrations
- bonded atoms behave as though they were connected by a spring, and are
therefore free to oscillate in space. This type of motion is called vibration, and it
results in stretching of bonds and deformation of the molecule’s shape
- in general, molecules possible to undergo two types of movement:
-depends on
molecular geometry,
bond lengths, and
bond angles.
translation
of the entire
molecule
rotation about
an axis
modes of vibration
12
Number of Vibration Modes
nonlinear
linear
translation
rotation
- a non-linear molecule of N atoms has 3N-6 normal modes of vibration; a linear
molecule has 3N-5.
- for a diatomic molecule, this means there will be only one vibration mode.
13
Example of Vibration modes for simple
molecules
Consider Sulfur Dioxide triatomic molecule that will have three fundamental or normal
modes of vibration:
n 1 = Symmetric
stretch
n3
n2
= Asymmetric
stretch
= Bend
(Scissoring)
Example of more complex molecule:
Square planar
Octrahedral
T-shaped
Pentagonal bipyramidal
Tetrahedral
3. Instrumentation
Raman Instrumentation
Source
Sample Illumination
Spectrometer
Simplified diagram of how a Raman spectrometer
works:
A sample is irradiated with monochromatic
laser light; which is then scattered by the
sample. The scattered light passes through a
filter to remove any stray light that may have
also been scattered by the sample. The filtered
light is then dispersed by the diffraction grating
and collected on the detector.
Lasers (Light amplification by
stimulated emission of radiation)
Type and wavelength of
laser source
Raman Instrumentation
Sample Illumination
Source
Spectrometer
Pinhole Aperture
Confocal Set Up
eliminates any image degrading
out-of-focus information,
allows for controllable depth of
field and gives the ability to
collect series of optical sections
Detector
Out of Focus Light Rays
Barrier Filter
filters and transmit both Stokes and
anti-Stokes Raman signals while
blocking the laser line
In Focus Light Rays
Laser
Dichroic Mirror
very accurate color
filter used to selectively
pass light of a small
range of colors while
reflecting other colors
Objective
Pinhole Aperture
Band Pass Filter - reduces scattered
- blocks all but
(stray) light and
the laser line
immensely improves
of interest
the image quality
Focal Planes
Sample
Raman Instrumentation
Source
Spectrometer
Sample Illumination
- used to measure properties of light over a specific portion of the electromagnetic spectrum
to identify materials.
Focusing Mirror
Collimating Mirror
- to produce parallel beams of
radiation, it overcomes diffraction
- reforms image from slit
onto focal plane
Dispersive
Grating
- Disperses radiation
into its component
wavelengths
.
Detector
Scattered Light
from Sample
4. Examples
A special attention is
taken for the S1 shoulder
analysis, due to the fact
that cubic-ZnS has a peak
close to 350 cm-1
Raman Spectra for CZTS thin film using different excitation laser wavelengths.
-The main peak of CZTS, P1, is located at 338-339 cm-1 and it is the strongest
peak at all excitation wavelengths.
-This is strong evidence that CZTS with the kesterite/stannite structure is the
dominant phase present.
-The second peak of CZTS, P2, at 287-288 cm-1
-The third peak of CZTS, P3, located at 367-368 cm-1