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THE SPECTRO-PAEDIA

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The Raman Scattering Process

If any substance is exposed to radiation of a defined frequency, then some light will be scattered at right angles. There are a number of different types of scattering. If the excitation energy is (hν0), and the molecule is raised from ground state to an excited level and then falls back to the ground state, the frequency of the light emitted will be the same as that of the incident light and this scattering phenomenon is called Rayleigh scattering. If the light is scattered by particulate matter as by a suspension of solids or by an emulsion the frequency of the light emitted will also be the same as the incident light and the scattered light but this is called Tyndale Scattering. In contrast, the accepted mechanism of Raman scattering is different to Rayleigh or Tyndale scattering but it is similar, though not exactly the same, as Fluorescence.

In Raman Scattering a molecule absorbs the incident radiation and, as a consequence, is raised to a higher level of energy. The molecule then emits light at the Raman frequency and falls to a new level, usually somewhere between the initial and final states. The scattered light will contain frequencies that differ from that of the incident light and, in addition, the frequencies of the scattered light will be characteristic of the substance that causes the scattering. The different forms of light scattering are depicted diagrammatically in figure 1.

Figure 1. Different Forms of Light Scattering

If the frequency of the incident light is (νo) and that of the scattered light (ν2) then the Raman frequency (Δν) is given by.

Δν = ν0 - ν2(1)

If, however, the light excites the molecule from ground level and then falls back to an energy level of (hν1) then the frequency of the emitted light will (ν0 ν1), and this is called Stokes–Raman scattering.

In Stokes–Raman Scattering, the scattered light always has a frequency lower than that of the incident light.

Howeer, if a molecule that already exits at a raised energy level of (hν3), is raised further to an excited state by the incident light and then falls back to the ground state, the frequency of the emitted light will now be (ν0 + ν3). This is called Anti-Stokes-Raman scattering, and the light scattered by this process always has a frequency greater than that of the incident light.

In Anti-Stokes–Raman Scattering, the scattered light always has a frequency higher than that of the incident light.copy there is a mechanism by which the incident radiation (normally its electric vector) interacts with the molecular energy levels. Infrared adsorption is associated with molecular vibrational levels and is detected by the changes in the dipole moment of the molecule as it vibrates. As a consequence it is very sensitive to polar functional groups. In contrast, Raman Spectroscopy discloses the presence of non-polar bonds and aromatic rings. It also discloses changes in polarization and the 'shape' of the electron distribution in the molecules as it vibrates. The information provided by the two techniques, Raman Spectroscopy and infrared spectroscopy, is often claimed to be complementary. Analysis of the Raman spectra from a wide range of compounds has shown that (hΔν) is almost invariably equal to the change in rotational or vibrational energy of the molecule. If the energy levels are close to the ground state, these levels will have a significant population as determined by the Boltzmann distribution and, thus, be observable.

The subject of Raman Scattering can be treated rationally by means of the quantum theory. However the following treatment is not entirely adequate nor is it complete but it is sufficiently so, to allow a basic understanding of the process of Raman Scattering

Radiation of frequency (ν) can be treated as a stream of particles having energy (hν) where (h) is Planks constant. If the photon striking a molecule is considered as a ball striking a solid surface, the ball will bounce off the surface with the same energy that it had when it struck it and, thus, the photon will have the same energy and the scattered radiation will have the same frequency. However, the collision may not be elastic and, thus, the photon can gain or loose energy and the scattered light may have a greater of lesser frequency than the incident radiation.

Thus, if (ΔE) is the energy change, the incident radiation has a frequency (ν) and the scattered frequency is (ν1),

Then, ν1 = ν ± ΔE/h

As already discussed, if the frequency of the scattered radiation is lower than the excitation radiation the light it is referred to as Stokes Radiation if the frequency is higher than the excitation radiation it is referred to as anti-Stokes radiation.

When a molecule is situated in an electric field (e.g. the undulating electric field portion of an electromagnetic wave) the molecule becomes polarized as the positive charges in the molecule are displaced in one direction and the negative charges in the opposite direction giving the molecule a transient dipole moment and, consequently, the molecule is said to be polarized.

If the magnitude of the induced dipole is (μ) and the electric field (E) then,

μ = αE

where (α) is the polarizability of the molecule

For radiation of frequency (ν) the field (E) associated with it will be given by,

E = EoSin2πνt

Thus, for the induced dipole. μ = αE = α EoSin2πνt

This will constitute an oscillating dipole of frequency (ν) that itself will generate radiation also of frequency (ν), which explains the source of Rayleigh scattering.

Any rotational or vibrational change in the molecule will also periodically change the polarizability of the molecule and, thus, the dipole will have a vibrational and/or rotational oscillation superimposed on it.

Thus for a vibrational frequency of (νvib) its polarizability will be given by,

α = αo + β2πνvibt

where (αo) is the equilibrium value of (α),

and (β) is the change in polarizability with external vibration.

Thus, μ = αE = (αo + βsιν2πνvibt)Eosin2πνt

Consequently as sinAsinB = [cos(A-B)- cos(A+B)}/2

and μ = αo EoSin2πνt + βEo sιν(2πνvibt)sin(2πνt)

μ = αo EoSin2πνt + βEo{cos2π(ν−νvib)t- cos2π(ν+νvib)t}/2

Consequently, the oscillating dipole contains frequencies of ν±νvib as well as the base frequency (ν).

However, if molecular vibration and/or molecular rotations do not affect the molecules polarizability then (β = 0) and the scattered radiation will have the same frequency as the exciting radiation and the scattering will not be Raman but Rayleigh scattering.

It should be noted that the previous statement is in contrast to the situation with infrared and microwave absorption where molecular motion must produce a change in the electric dipole of the molecule. When a particular bond between atoms produces a strong infrared signal, it is less likely to produce a strong Raman signal, and vice versa.

The study of Raman spectra has a number of advantages as, by the appropriate choice of the incident radiation, the scattered lines can be brought into a convenient region of the Spectrum where they can be easily observed. The energy of the incident radiation determines the spectroscopic region where the Raman Scattering is observed. Originally both incident and scattered radiation were measured in the Visible region of the Spectrum but, for various reasons, one of which is discussed below, the near-infra-red radiation is now the most frequently employed.

Initially, the practice of Raman Spectroscopy was experimentally more difficult than today, as the intensity of the scattered light is very small (only 0.0001% of that of the incident light, i.e. one part in a million). The difficulties were greatly reduced with the introduction of the laser light sources of high energy in the 1960s and, in particular, the argon laser with its intense blue and green emission. Nevertheless, although the high-intensity laser light sources has aided in Raman spectroscopic techniques, it has also led to other problems, such as photochemical reaction in the sample and sample heating with resultant black-body radiation. Another serious problem associated with practical Raman Spectroscopy is the Fluorescence that can often accompany the Raman Scattering that can be as much as six to eight orders of magnitude stronger then the Raman light.

Often, when trying to examine Raman Scattering, Fluorescence is the only phenomenon observed.

The Fluorescence can come from a variety of sources. It is often caused by trace impurities, coatings on polymers , additives etc., that provide so much background Fluorescence that the Raman Spectrum of the major component cannot be discerned. The use of near infrared excitation can help solve this problem. It has been found, that the use of light having a wavelength around 1 μm to irradiate the sample virtually eliminates the fluorescent problem. However, other problems remain such as photochemical changes in the sample and blackbody radiation produced by local heating. These problems must be carefully distinguished from Fluorescence, as the experimental procedure necessary to correct for these effects differ greatly from the precautions that need to be taken against Fluorescence.

The Raman Spectrum of a given substance measured under controlled conditions can be used to confirm the identity of the substance. An example of the different absorption curves for infrared and Raman Spectroscopy is shown in figure 2.

Courtesy of Nicolet Inc.

Figure 2. Infrared and Raman Spectra of Some Gasoline's of Different Octane Ratings.

It is seen that the spectra for the gasoline's having different Octane rating are basically similar but, although the IR spectra show minimal differences between the samples, there are clear and unambiguous differences in the Raman spectra. In figure 3 spectra are shown that have been taken from aspirin powder by Diffuse Reflectance infrared spectroscopy and Raman Spectroscopy.

Courtesy of the Perkin Elmer Corporation

Figure 3. IR Diffuse Reflectance and Raman Spectra of aspirin Powder

It is seen that absorbance and Raman Scattering takes place at very similar wavelengths and there is little to choose between the two spectra for substance identification. However if the sample is in another form, e.g. as the unprepared tablets themselves, the spectra differ considerably, as shown in figure 4.


Courtesy of the Perkin Elmer Corporation

Figure 4. IR Diffuse Reflectance and Raman Spectra of an aspirin Tablet

It is seen that the spectra are now quite different; the infrared Spectrum is virtually useless whereas the Raman Spectrum remains similar to that obtained from the powder. One of the great advantages of Raman Spectroscopy is that it is virtually independent of the form that the sample takes. This could be a benefit that should be taken into account when assessing the best spectroscopic technique to be used. Unfortunately, the technique is even less sensitive than IR spectroscopy so its value as an identifying technique can sometimes be rather limited. Nevertheless, the use of a laser light source in conjunction with FT/IR will help in this respect


 

About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.

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