Raman spectroscopy - material de apoio

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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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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. 
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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.
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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
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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. 
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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. 
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First Report of Raman Observation
	Nature 121, 501-502 (31 March 1928)
	A New Type of Secondary Radiation
	C. 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. 
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First Report of Raman Observation
	Nature 121, 501-502 (31 March 1928)
	A New Type of Secondary Radiation
	C. V. RAMAN &  K. S. KRISHNAN 
	 Continue
	The 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.
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The Nobel Prize in Physics 1930
	"for his work on the scattering of light and for the discovery of the effect named after him"
http://nobelprize.org/nobel_prizes/physics/laureates/1930/raman-lecture.pdf
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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)	
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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.
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Stokes and Anti-Stokes Scattering
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Stokes and Anti-Stokes Scattering
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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… 
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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. 
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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.
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Raman Activity of H2O Vibrations
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Raman Activity of CO2 Vibrations
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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. 
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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.
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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
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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. 
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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.
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 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)