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Raman Effect

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Information about Raman Effect
Education

Published on December 27, 2013

Author: yogeshkumarchauhan

Source: slideshare.net

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 November 7, 1888 - November 21, 1970  Won the Nobel prize in 1930 for Physics.  Raman effect was first reported by Sir CV Raman and KS Krishnan.  Besides discovering the raman effect he studied extensively in X-ray diffraction, optics , dielectrics and colloidal solutions.

   Only objects whose dimensions are on the order of ~11.5 will scatter EM radiation (molecules). Rayleigh scattering : the scattering of light by particles in a medium, without change in wavelength. Raman scattering : The scattering of radiation with change of frequency is called Raman scattering. RAMAN SPECTROSCOPY is a technique that uses scattered light resulting from photon–molecule collisions to investigate molecular properties.

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

Light is a travelling wave of electric and magnetic fields  only the electric component gives rise to Raman scattering.  When a light wave meets a molecule consisting of electrons and nuclei, the electric field of the wave at any instant will be the same throughout the molecule.  the field will exert the same force on all electrons in the molecule and will tend to displace them.  the displacements result in an induced dipole moment, μ(ind) in the molecule. Thus μ(ind) = αE 

   α is called the electric polarizability of the molecule. The Raman interaction leads to two possible outcomes: If the material absorbs energy and the emitted photon has a lower energy than the absorbed photon. This is Stokes Raman scattering while for antistockes raman scattering reverse is true. The spectrum of the scattered photons is the Raman spectrum. It shows the intensity of the scattered light as a function of its frequency difference Δν to the incident photons.

The Raman effect comprises a very small fraction, about 1 in 107 of the incident photons

A Raman spectrum is a plot of the intensity of Raman scattered radiation as a function of its frequency difference from the incident radiation (usually in units of wave numbers, cm⁻¹). This difference is called the Raman shift.

A modern Raman spectrometer

 Identification of compound by matching the spectrum of unknown with reference spectrum (fingerprint).    Determination of molecular composition of surfaces. Identification of all type of inorganic compounds in solid and aqueous solution. Detection of molecular impurites and additives.

     Advantages of Raman over IR: Avoids many interferences from solvents, cells and sample preparation methods. Depolarization studies possible, enhanced effects in some cases. Can detect IR-inactive vibrational modes. Advantages of IR over Raman:  Raman lines are weaker, the Rayleigh line is also present. Raman instruments can be more costly. Raman can suffer from laser-induced fluorescence and degradation.  Final conclusion : Both techniques are complementary.  

 Experiments done in the mid 1970s demonstrated that molecules adsorbed to a roughened metal surface generated large Raman intensities. This is known as surface-enhanced Raman spectroscopy (SERS).  SERS is a form of Raman spectroscopy that involves a molecule adsorbed to the surface of a nanostructured metal surface.

 Raman intensity lines are 0.001% (at most) of the source intensity.  The intensity can be increased by 10³ – 106 orders of magnitude if the sample is adsorbed on the surface of colloidal metal particles.

 The mechanism of SERS is not completely understood.  EM Enhancement  Proposed by Jeanmarie and Van Duyne in 1977.  Chemical enhancement  Proposed by Albrecht and Creighton in 1977.

NanoRaman…………… To further expand the capabilities of a Raman system, some manufacturers offers coupling solutions to other analytical techniques. Coupling to other non-optical imaging techniques like AFM enables to go far beyond the diffraction limit, while keeping the chemical information that Raman spectroscopy brings.

Stanford scientists create a new way to scan for tumors - a process that is potentially safer and more sensitive than current cancer process.  beam of laser light projected on the skin surface, technique can spot with extraordinary precision tiny particles that have been injected into the bloodstream and attach themselves to cancer cells.  PET scans, can pick up a tumor about 5 mm wide containing tens of millions of cancerous cells while raman imaging has the potential to detect microscopic lumps of only a few hundred cancer cells." 

Thank you…..

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