CHEM 333. RAMAN SPECTROSCOPY. Rabia Aslam Chaudary 2012-10-0011. OUTLINE: Introduction Explanation of Vibrational Spectrum Quantum explanation Examples Applications Advantages and Disadvantages Future prospects. INTRODUCTION:
Raman spectroscopy is a valuable tool for the characterization of materials due to its extreme sensitivity to the molecular environment of the species of interest.
Information on molecular vibrations can provide much structural, orientational, and chemical information that can assist in defining the environment of the molecule of interest to a high degree of specificity.
The Raman effect is named after C.V. Raman, who, with K.S. Krishnan, first observed this phenomenon in 1928. It belongs to the class of molecular-scattering phenomena.
A line (band) that is part of a Raman spectrum and corresponds to a characteristic vibrationalfrequency of the molecule being probed.
2. Raman shift
The displacement in wave number of a Raman line (band) from the wave number of the incidentmonochromatic beam. Raman shifts are usually expressed in units of cm-1. They correspond todifferences between molecular vibrational, rotational, or electronic energylevels.
3. Raman spectroscopy
Analysis of the intensity of Raman scattering of monochromatic light as a function of frequency ofthe scattered light.
Because anti-Stokes scattering can occur only for molecules that are in an excited vibrational or rotational state before scattering, the intensity of anti-Stokes radiation is significantly less than that of Stokes radiation at room temperature.
Therefore, Raman spectroscopy generally uses Stokes radiation. Overall, however, the total amount of inelastically scattered Stokes and anti-Stokes radiation is small compared to the elastically scattered Rayleigh radiation.
A molecule in an electromagnetic field is distorted by the electrons to the positive pole of the electric field and attraction of nuclei to the negative pole of the field. The extent of distortion is called polarizibility.
Separation of charges produce a dipole and dipole moment per unit volume is known as polarization.
The oscillating Electric field is and consequently polarization would be:
Now, an oscillating dipole moment is a source of radiation.
If molecules undergo internal motion like vibration or rotation then polarizibility changes periodically.
This equation predicts three components of scattered radiation!
In the study of metal oxides systems, Raman overcomes the problem with infrared analysis which is the interference from absorption of the radiation by the underlying bulk material. These species are strong to infrared but only weak to moderate Raman scatters. Metal oxides exhibit strong absorption in the infrared region.
Another example would be of pyridine. Its ring-breathing vibrational modes are extremely sensitive to the chemical environment.
The weak Raman scattering of water makes the Raman analysis of polymers in aqueous media particularly attractive. \One of the major advantages of Raman spectroscopy is the availability of the entire vibrational spectrum using one instrument.
Raman spectroscopy has been used extensively to characterize the extent of surface structural disorder ingraphites. Graphites is experimentally complicated by strong absorption of laser radiation, which damages the surface significantly. To avoid surface decomposition during analysis, low laser powers on a stationary graphite sample are used.
Now-a-days Raman spectroscopic technology is utilized for the detection of changes occurring at the molecular level during the pathological transformation of the tissue. The potential of its use in urology is still in its infancy and increasing utility of this technology will transform noninvasive tissue diagnosis.
It has shown encouraging results in diagnosis and grading of the bladder and the prostate. Raman microprobes have been used for the characterization and identification of renal lithiasis.
The future would see the development of optical fiber probes to incorporate them into catheters, endoscopes, and laparoscopes that will enable the urologist to obtain information during the operation.
Raman spectroscopy is an emerging technique that is able to interrogate biological tissues, that gives us an understanding of the changes in molecular structure that are associated with disease development. The potential applications of Raman spectroscopy may herald a new future in the management of various malignant, premalignant, and other benign conditions in urology.
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