RAMAN SPECTROSCOPY

CHEM 333

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.

Energy Range (eV) λ

Electronic 8.28 – 0.828 150 – 1500 nm

Vibrational 0.828 – 0.0124

1.5 – 100 µm

Rotational 1.24 X 10-2 – 1.24 X 10-4

100 – 10,000 µm

Light scattering effects:

• A radiation which consists of photons undergoes undergo two types of collisions with the molecules of a medium.

• Elastic Collisions: in which scattered photon has same energy as incident photon. (Rayleigh scattering)

• Inelastic Collisions: in which scattered photon has more (Anti-stokes Raman scattering) or less energy(Stokes Raman scattering) as incident photon.

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1. Raman line (band)A line (band) that is part of a Raman spectrum and corresponds to a characteristic vibrational frequency of the molecule being probed.

2. Raman shiftThe displacement in wave number of a Raman line (band) from the wave number of the incident monochromatic beam. Raman shifts are usually expressed in units of cm-1. They correspond to differences between molecular vibrational, rotational, or electronic energy levels.

3. Raman spectroscopyAnalysis of the intensity of Raman scattering of monochromatic light as a function of frequency of the scattered light.

4. Raman spectrum

The spectrum of the modified frequencies resulting from inelastic scattering when matter is irradiated by a monochromatic beam of radiant energy. Raman spectra normally consist of lines (bands) at frequencies higher and lower than that of the incident monochromatic beam.

•Anti-Stokes Raman line: A Raman line that has a frequency higher than that of the incident monochromatic radiation

•Stokes Raman line : A Raman line that has a frequency lower than that of the incident monochromatic beam.

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.

Quantum mechanics Explanation:

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 magnitude of polarization is given by:

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!

APPLICATIONS:

• Use of laser as an intense monochromatic light that overcomes weak Raman scattering and exploits the advantages of Raman spectroscopy in the vibrational analysis of surfaces and surface species overcoming the limitations of infrared spectroscopy.

• Raman spectroscopy is accessible to the low-frequency region of the spectrum thus giving a complete vibrational analysis of surface species.

• It is also valuable in probing surface processes in aqueous environments due to extreme weakness of Raman scattering in water. This has made study of corrosion possible where Raman scattering being weak in water does not interfere with detection of metal corrosion products.

• 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.

• Raman spectroscopy has been extensively used to characterize the surface structure of supported metal oxides. Example includes Raman investigation of chemical species formed during calcination and activation of tungsten trioxide catalysts supported on Silica and Alumina. Results show that crystalline and polymeric forms of Tungsten trioxide are present on Silicon dioxide supported surfaces.

• Another example would be of pyridine. Its ring-breathing vibrational modes are extremely sensitive to the chemical environment.

The various interactions produce substantial shifts in the peak frequency of symmetrical ring-breathing vibration of pyridine. In general, the more strongly interacting the lone pair of electrons on the pyridine nitrogen, the larger the shift to higher frequencies.

MODEL COMPOUND

V1/cm Nature of interaction

Pyr 991 Neat Liquid

Pyr in CHCl3 998 H-bond

Pyr in CH2Cl2 992 No interaction

Pyr in CCl4 991 No interaction

Pyr in H2O 1003 H-bond

Pyr:ZnCl2 1025 Coordinately bound

Pyr N-Oxide 1016 Coordinately bound

Pyr: GaCl3 1021 Coordinately bound

• 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.

• The dependence of Raman scattering intensity on changes in polarizability of molecule makes it particularly sensitive to symmetrical vibrations and to vibrations involving larger atoms. In particular, Raman intensities are more sensitive than infrared for the detection of C=C, C C, phenyl, C-S, and S-S vibrations various types of degradation and polymerization have involved observing changes in vibrational features that affect only a small part of the polymer.

Raman spectroscopy has been used extensively to characterize the extent of surface structural disorder in graphites. 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.

Raman microprobe analysis has been used with transmission electron microscopy (TEM) to characterize vapor deposition of carbon films on alkali halide cleavage.

Raman spectroscopy has also provided evidence for the intercalation of Br2, ICl, and IBr as molecular entities.

Applications in Medicine:

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.

•Can be used with solids and liquids•No sample preparation needed•Not interfered by water•Non-destructive•Highly specific like a chemical fingerprint of a material•Raman spectra are acquired quickly within seconds•Samples can be analyzed through glass or a polymer packaging•Laser light and Raman scattered light can be transmitted by optical fibers over long distances for remote analysis•Raman spectra can be collected from a very small volume (< 1 µm in diameter)•Inorganic materials are normally easily analyzed by Raman than by infrared spectroscopy

•Can not be used for metals or alloys.•The Raman effect is very weak. The detection needs a sensitive and highly optimized instrumentation•Fluorescence of impurities or of the sample itself can hide the Raman spectrum•Sample heating through the intense laser radiation can destroy the sample or cover the Raman spectrum

ADVANTAGES DISADVANTAGES

FUTURE OF RAMAN SPECTROSCOPY:

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.

REFERENCES:

1. C.V. Raman and K.S. Krishnan, Nature, Vol 122, 1928, p 5012. D.A. Long, Raman Spectroscopy, McGraw-Hill, 19773. M.C. Tobin, Laser Raman Spectroscopy, John Wiley & Sons, 19714. D.C. O' Shea, W.R. Callen, and W.T. Rhodes, Introduction to Lasers and Their

Applications,5. Addison-Wesley, 19776. Spex Industries, Metuchen, NJ, 19817. E.E. Wahlstrom, Optical Crystallography, 4th ed., John Wiley & Sons, 19698. M. Delhaye and P. Dhemalincourt, J. Raman Spectrosc., Vol 3, 1975, p 339. P. Dhamelincourt, F. Wallart, M. Leclercq, A.T. N'Guyen, and D.O. Landon, Anal. Chem.,

Vol 51, 1979,p 414A10. P.J. Hendra and E.J. Loader, Trans. Faraday Soc., Vol 67, 1971, p 82811. E. Buechler and J. Turkevich, J. Phys. Chem., Vol 76, 1977, p 2325

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