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.

12

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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