1. Raman Spectroscopy
Lord Rutherford, 1930
April 10th, 2014

2. 1. Introduction

3. 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
In the past, the word spectrum was introduced into the area of optics at first, referring to the range of colors observed when white light was dispersed through a prism. Soon after that, a spectral density or spectrum is also known as the term referred to a plot of light intensity or power as a function of frequency or wavelength. Spectroscopy is the study of spectrum, study of the interaction between radiation , for identifying the substances through the spectrum emitted from or absorbed by them.
complicated

4. Sir Chandrasekhara Venkata Raman
Who is Raman?
Sir Chandrasekhara Venkata Raman
( )
Raman was awarded the Nobel Prize in Physics in 1930 for his discovery.
In 1921 Raman took a tour to Europe as a delegate of Universities’ congress
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

5. 2. Theory

6. 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)

7. Classical Mathematical Argument:
Consider the time variation of the dipole moment induced by incident radiation (an EM field):
*An electric field applied to a molecule results in its distortion, and the distorted molecule acquires a contribution to its dipole moment
Induced dipole moment
EM field
polarizability
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:
mean polarizability
 = max - min
Note that this is a classical argument, see Atkins and Friedman
Expanding this product yields:
Rayleigh line
Anti-Stokes line
Stokes line

8. Stokes/Anti-Stokes Schematic diagram of the process:
Note that the transitions (scattering) take 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.

9. Stokes/Anti-Stokes Animation
Energy transferred from incident light to molecular vibrations 3 2 1 S0 hn
Inelastic Scattering
h(n (-+) n1)
Virtual Level hn
Energy
difference in energy
Rayleigh Raman (inelastic)
(elastic) Scattering Scattering

10. 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
Stokes Shift
Anti-Stokes Shift
100
200
300
-300
-200
-100
Raman Shift (cm-1)
Intensity
-on this scale, band positions will lie at frequency that correspond to energy level of different functional group.
-the Stokes lines are stronger because the population of molecules at =0 is much larger than at
=1 by the Maxwell-Boltzmann distribution law.
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

11. 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.

12. 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

13. 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.

14. Example of Vibration modes for simple molecules
Consider Sulfur Dioxide triatomic molecule that will have three fundamental or normal modes of vibration:
= Symmetric
stretch
= Bend
(Scissoring)
= Asymmetric

15. Example of more complex molecule:
Square planar
Octrahedral
T-shaped
Pentagonal bipyramidal
Tetrahedral

16. 3. Instrumentation

17. 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

18. Raman Instrumentation
Source Sample Illumination 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
Barrier Filter
Out of Focus Light Rays
filters and transmit both Stokes and anti-Stokes Raman signals while blocking the laser line
In Focus Light Rays
Laser
Dichroic Mirror
Objective
very accurate color filter used to selectively pass light of a small range of colors while reflecting other colors
Pinhole Aperture
Band Pass Filter
- reduces scattered (stray) light and
immensely improves the image quality
- blocks all but the laser line of interest
Focal Planes
Sample

19. Raman Instrumentation
Source Sample Illumination Spectrometer
- used to measure properties of light over a specific portion of the electromagnetic spectrum to identify materials.
Focusing Mirror
Collimating Mirror
- reforms image from slit onto focal plane
- to produce parallel beams of radiation, it overcomes diffraction
Dispersive Grating
- Disperses radiation into its component wavelengths .
Scattered Light from Sample
Detector

20. 4. Examples

24. peak at all excitation wavelengths.
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 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 cm-1
-The third peak of CZTS, P3, located at cm-1

27. Thank You For Your Attention