April 10th, 2014
Raman Spectroscopy
Lord Rutherford, 1930
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?· 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.
· Raman was awarded the Nobel Prize in Physics in 1930 for his discovery.
· 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
Raleigh Scatter (same wavelength as incident light)
Raman Scatter (new wavelength)
Sample
Incident Laser
Scattered Light
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.
· Consider the time variation of the dipole moment induced by incident radiation (an EM field):
)()()( ttt EM fieldInduced dipole moment
· Expanding this product yields:
tttt )cos()cos(cos)( intint041
0
Rayleigh line Anti-Stokes line Stokes line
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:
ttt coscos)( 0int21
mean polarizability = max - min
*An electric field applied to a molecule results in its distortion, and the distorted molecule acquires a contribution to its dipole moment
Classical Mathematical Argument:
Stokes/Anti-StokesNote that the transitions (scattering) take 10-14 seconds or less!
Schematic diagram of the process:
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.
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.
hn
h( (n -+) n1)
hn
3
2
1
0 S0
Ene
rgy
Virtual Level
Rayleigh Raman (inelastic)(elastic) Scattering Scattering
Inelastic Scattering
difference in energy
• Energy transferred from incident light to molecular vibrations
Stokes/Anti-Stokes Animation
Raman Spectrum
-the Stokes lines are stronger because the population of molecules at =0 is much larger than at =1 by the Maxwell-Boltzmann distribution law.
Rayleigh Scattering
Stokes Shift
Anti-Stokes Shift
0100200300 -300-200-100
Raman Shift (cm-1)
Inte
nsity
Ideal Raman spectrum:-Raman spectrum are plotted with respect of the laser frequency such that Rayleigh band lies at 0 cm-1
-on this scale, band positions will lie at frequency that correspond to energy level of different functional group.
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 SpectrumPractical 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.
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Molecular Vibrations
- in general, molecules possible to undergo two types of movement:
-depends on molecular geometry, bond lengths, and bond angles.
- 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
translation of the entire molecule
rotation about an axis
modes of vibration
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Number of Vibration Modes
translation
rotation
nonlinear linear
- 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.
Example of Vibration modes for simple molecules
2
3
1
= Symmetric stretch
= Bend (Scissoring)
= Asymmetric stretch
Consider Sulfur Dioxide triatomic molecule that will have three fundamental or normal modes of vibration:
Example of more complex molecule:
Octrahedral
Pentagonal bipyramidal
Square planar
T-shaped
Tetrahedral
3. Instrumentation
Raman Instrumentation
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.
Source Sample Illumination Spectrometer
Type and wavelength of laser source
Lasers (Light amplification by stimulated emission of radiation)
Raman Instrumentation
Source Sample Illumination Spectrometer
Confocal Set Up
Laser
Detector
Pinhole ApertureBand Pass Filter
Pinhole Aperture
Out of Focus Light Rays
In Focus Light Rays
Dichroic MirrorObjective
Sample
Focal Planes
Barrier Filter
- reduces scattered (stray) light andimmensely improves the image quality
eliminates any image degrading out-of-focus information,allows for controllable depth of field and gives the ability to collect series of optical sections
very accurate color filter used to selectively pass light of a small range of colors while reflecting other colors
- blocks all but the laser line of interest
filters and transmit both Stokes and anti-Stokes Raman signals while blocking the laser line
Raman Instrumentation
Source Sample Illumination Spectrometer
Detector
Collimating MirrorFocusing Mirror
Dispersive Grating
Scattered Light from Sample
- to produce parallel beams of radiation, it overcomes diffraction
- Disperses radiation into its component wavelengths.
- used to measure properties of light over a specific portion of the electromagnetic spectrum to identify materials.
- reforms image from slit onto focal plane
4. Examples
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 strongestpeak 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
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
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