raman spectroscopy

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Raman Spectroscopy Prof. V. Krishnakumar Professor and Head Department of Physics Periyar University Salem – 636 011, India

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An introduction to Raman Spectroscopy

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Page 1: Raman Spectroscopy

Raman Spectroscopy

Prof. V. KrishnakumarProfessor and Head

Department of PhysicsPeriyar University

Salem – 636 011, India

Page 2: Raman Spectroscopy

What is Spectroscopy?• The study of how 'species' (i.e.,

atoms, molecules, solutions) react to light. Some studies depend on how much light an atom absorbs. The electromagnetic radiation absorbed, emitted or scattered by the molecule is analyzed. Typically, a beam of radiation from a source such as a laser is passed through a sample, and the radiation exiting the sample is measured. Some, like Raman, depend on a molecule's vibrations in reaction to the light.

Page 3: Raman Spectroscopy

Light Scattering Phenomenon

• When radiation passes through a transparent medium, the species present in that medium scatter a fraction of the beam in all directions.

Page 4: Raman Spectroscopy

Raman Effect (or Raman Scattering)

• 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.

• Furthermore, he noted that the change (shifts) in frequency depend upon the chemical structure of the molecules responsible for the scattering

First photographed Raman spectra

Page 5: Raman Spectroscopy

Why Raman?• In Raman spectroscopy,

by varying the frequency of the radiation, a spectrum can be produced, showing the intensity of the exiting radiation for each frequency. This spectrum will show which frequencies of radiation have been absorbed by the molecule to raise it to higher vibrational energy states.

Page 6: Raman Spectroscopy

What Exactly Is Being Measured?METHANE

When Light hits a sample, It is Excited, and is forced to vibrate and move. It is these vibrations which we are measuring.

Page 7: Raman Spectroscopy

First Report of Raman ObservationNature 121, 501-502 (31 March 1928)

A New Type of Secondary RadiationC. V. RAMAN & K. S. KRISHNAN

Abstract

If we assume that the X-ray scattering of the ‘unmodified’ type observed by Prof. Compton corresponds to the normal or average state of the atoms and molecules, while the ‘modified’ scattering of altered wave-length corresponds to their fluctuations from that state, it would follow that we should expect also in the case of ordinary light two types of scattering, one determined by the normal optical properties of the atoms or molecules, and another representing the effect of their fluctuations from their normal state. It accordingly becomes necessary to test whether this is actually the case. The experiments we have made have confirmed this anticipation, and shown that in every case in which light is scattered by the molecules in dust-free liquids or gases, the diffuse radiation of the ordinary kind, having the same wave-length as the incident beam, is accompanied by a modified scattered radiation of degraded frequency.

Page 8: Raman Spectroscopy

First Report of Raman Observation

Nature 121, 501-502 (31 March 1928)

A New Type of Secondary RadiationC. V. RAMAN & K. S. KRISHNAN

ContinueThe new type of light scattering discovered by us naturally requires very powerful illumination for its observation. In our experiments, a beam of sunlight was converged successively by a telescope objective of 18 cm. aperture and 230 cm. focal length, and by a second lens was placed the scattering material, which is either a liquid (carefully purified by repeated distillation in vacuo) or its dust-free vapour. To detect the presence of a modified scattered radiation, the method of complementary light-filters was used. A blue-violet filter, when coupled with a yellow-green filter and placed in the incident light, completely extinguished the track of the light through the liquid or vapour. The reappearance of the track when the yellow filter is transferred to a place between it and the observer's eye is proof of the existence of a modified scattered radiation. Spectroscopic confirmation is also available.

Page 9: Raman Spectroscopy

The Nobel Prize in Physics 1930

"for his work on the scattering of light

and for the discovery of the

effect named after him"

Professor Sir C.V. Raman

1888-1970

http://nobelprize.org/nobel_prizes/physics/laureates/1930/raman-lecture.pdf

Page 10: Raman Spectroscopy

Rayleigh Scattering and Raman Scattering

The frequency of the scattered light can be:

• at the original frequency (νI) “Rayleigh scattering” very strong.

• at some shifted frequency

(νs = νI ± νmolecule) “Raman scattering or Raman shift” very weak (~ 10-5 of the incident beam)

Page 11: Raman Spectroscopy

Stokes and Anti-Stokes Scattering

• Raman shift can correspond either to rotational, vibrational or electronic frequencies.

Δν = |νI – νs|

• Radiation scattering to the lower frequency side (to the red side) of the Rayleigh line is called Stokes scattering.

• Radiation scattering to the higher frequency side (to the blue side) of the Rayleigh line is called anti-Stokes scattering.

Page 12: Raman Spectroscopy

Stokes and Anti-Stokes Scattering

Page 13: Raman Spectroscopy

Stokes and Anti-Stokes Scattering

Page 14: Raman Spectroscopy

Number of bands in a Raman spectrum

As for an IR spectrum, the number of bands in the Raman spectrum for an N-atom non-linear molecule is seldom 3N-6, because:

polarizability change is zero or small for some vibrations;

bands overlap;

combination or overtone bands are present;

Fermi resonances occur;

some vibrations are highly degenerate; etc…

Page 15: Raman Spectroscopy

Nature of Polarizability

Polarizability is the relative tendency of a charge distribution, like the electron cloud of an atom or molecule, to be distorted from its normal shape by an external electric field, which may be caused by the presence of a nearby ion or dipole or by an applied external electric field.

Page 16: Raman Spectroscopy

Raman Activity of Molecular Vibrations

• In order to be Raman active, a molecular rotation or vibration must cause some change in a component of the molecular polarizability. The change can either be in the magnitude or the direction of the polarizability ellipsoid.

• Polarizability ellipsoid is a three-dimensional body generated by plotting 1/√α from the center of gravity in all directions.

• This rule must be contrasted with that for IR activity that requires change in the net dipole moment of the molecule.

Page 17: Raman Spectroscopy

Raman Activity of

H2O Vibrations

Page 18: Raman Spectroscopy

Raman Activity of

CO2 Vibrations

Page 19: Raman Spectroscopy

Raman and Infrared are Complementary Techniques

• Interestingly, although they are based on two distinct phenomena, the Raman scattering spectrum and infrared absorption spectrum for a given species often resemble one another quite closely in terms of observed frequencies.

The infrared and Raman spectrum of styrene/buta-diene rubber.

Page 20: Raman Spectroscopy

Rule of Mutual Exclusion

• If a molecule has a center of symmetry, then Raman active vibrations are infrared inactive, and vice versa. If there is no center of symmetry, then some (but not necessarily all) may be both Raman and infrared active.

Page 21: Raman Spectroscopy
Page 22: Raman Spectroscopy

• FT-Raman

• Fluorescence-free Raman spectra by 1064nm excitation

• Simple measurement of bulk samples due to advantage of sample compartment

• Quantification

• Dispersive Raman

• Better spatial resolution for microscopy applications (down to 1µm)

• Higher sensitivity and shorter measurement times for non-fluorescing samples

• Selection of different excitation lines (488-785nm)

Comparison between FT and dispersive Raman

Page 23: Raman Spectroscopy

Uses of Raman Spectroscopy

Raman spectroscopy has become more widely used since the advent of FT-Raman systems and remote optical fibre sampling. Previous difficulties with laser safety, stability and precision have largely been overcome.

Basically, Raman spectroscopy is complementary to IR spectroscopy, but the sampling is more convenient, since glass containers may be used and solids do not have to be mulled or pressed into discs.

Page 24: Raman Spectroscopy

Applications of Raman spectroscopy

Qualitative tool for identifying molecules from their vibrations, especially in conjunction with infrared spectrometry.

Quantitative Raman measurements

a) not sensitive since Raman scattering is weak. But resonance Raman spectra offer higher sensitivity, e.g. fabric dyes studied at 30-50 ppb.

b) beset by difficulties in measuring relative intensities of bands from different samples, due to sample alignment, collection efficiency, laser power.

Overcome by using internal standard.

Page 25: Raman Spectroscopy

Raman vs IR spectroscopy

RAMAN IR

Sample preparation usually simpler

Liquid/ Solid samples must be free

from dust

Biological materials usually fluoresce,

masking scattering

Spectral measurements on vibrations Halide optics must be used-

made in the visible region-glass cells expensive, easily broken,

may be used water soluble

Depolarization studies are easily made IR spectrometers not usually

(laser radiation almost totally linearly equipped with polarizers

polarized)