Scattering The Raman Effect

Inelastic photon scattering processes are also possible. In 1928, the Indian scientist C. V. Raman (who won the Nobel Prize in 1930) demonstrated a type of inelastic scattering that had already been predicted by A. Smekal in 1923. This type of scattering gave rise to a new type of spectroscopy, Raman spectroscopy, in which the light is inelastically scattered by a substance. This effect is in some ways similar to the Compton effect, which occurs as a result of the inelastic scattering of electromagnetic radiation by free electrons. 

Most of the four above-mentioned properties for Raman spectra can be explained by using a simple classical model. When the crystal is subjected to the oscillating electric field = fioc of the incident electromagnetic radiation, it becomes polarized. In the linear approximation, the induced electric polarization in any specific direction is given by Pj = XjkEk, where Xjk is the susceptibility tensor. As for other physical properties of the crystal, the susceptibility becomes altered because the atoms in the solid are vibrating periodically around equilibrium positions. Thus, for a particular 

On the other hand, the radiated intensity of such oscillating dipoles is proportional to d T/dr f, so that we can write  

The first term on the right-hand side of Equation (1.24) accounts for the generated intensity due to Rayleigh scattered light, while the second term is related to the intensity of the Raman scattered light. For visible light coo 10 Hz, while the characteristic phonon frequencies are much shorter, typically 12 10 Hz. Then coq and the intensity of Raman scattering varies as coq, as stated in point (iv) above. 

In resonant Raman spectroscopy, the frequency of the incident beam is resonant with the energy difference between two real electronic levels and so the efficiency can be enhanced by a factor of 10 . However, to observe resonant Raman scattering it is necessary to prevent the possible overlap with the more efficient emission spectra. Thus, Raman experiments are usually realized under nonresonant illumination, so that the Raman spectrum cannot be masked by fluorescence. 

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It was predicted in 1923 by Smekal and shown experimentally in 1928 by Raman and Krishnan that a small amount of radiation scattered by a gas, liquid or solid is of increased or decreased wavelength (or wavenumber). This is called the Raman effect and the scattered radiation with decreased or increased wavenumber is referred to as Stokes or anti-Stokes Raman scattering, respectively. 

The incident radiation should be highly monochromatic for the Raman effect to be observed clearly and, because Raman scattering is so weak, it should be very intense. This is particularly important when, as in rotational Raman spectroscopy, the sample is in the gas phase. 

Spectroscopic examination of light scattered from a monochromatic probe beam reveals the expected Rayleigh, Mie, and/or Tyndall elastic scattering at unchanged frequency, and other weak frequencies arising from the Raman effect. Both types of scattering have appHcations to analysis. 

Barkla, originally interested mainly in v-ray scattering, discovered characteristic x-rays by an experimental method similar in principle to that described above. His experimental arrangement (Figure 1-7) is reminiscent of that used today in studies of the Raman effect. By using an absorber in the form of sheets (Figure 1-7) to analyze the scattered beam in the manner of Figure 1-4, he obtained results that clarified the earlier experiments described above. 

Since the Raman scattering is not very efficient (only one photon in 107 gives rise to the Raman effect), a high power excitation source such as a laser is needed. Also, since we are interested in the energy (wavenumber) difference between the excitation and the Stokes lines, the excitation source should be monochromatic, which is another property of many laser systems. 

In crystalline solids, the Raman effect deals with phonons instead of molecular vibration, and it depends upon the crystal symmetry whether a phonon is Raman active or not. For each class of crystal symmetry it is possible to calculate which phonons are Raman active for a given direction of the incident and scattered light with respect to the crystallographic axes of the specimen. A table has been derived (Loudon, 1964, 1965) which presents the form of the scattering tensor for each of the 32 crystal classes, which is particularly useful in the interpretation of the Raman spectra of crystalline samples. 

Figure 2.52 Schematic representation of the transitions giving rise to the Raman effect. GS = ground electronic state, ES = excited electronic state, VS = virtual electronic stale, R = Rayleigh scattering, S = transitions giving rise to Stokes lines, AS = transitions giving rise to Anti-Stokes lines, RRS = transitions giving rise to resonance Raman.

The Raman effect arises when a photon is incident on a molecule and interacts with the electric dipole of the molecule. In classical terms, the interaction can be viewed as a perturbation of the molecule s electric field. In quantum mechanics the scattering is described as an excitation to a virtual state lower in energy than a real electronic transition with nearly coincident de-excitation and a change in vibrational energy. The scattering event occurs in 10 14 seconds or less. The virtual state description of scattering is shown in Figure 1. 

Inelastic scattering of light due to the excitation of vibrations had already been predicted in 1923 [37] and was confirmed experimentally a few years later by Raman [38], Because at that time the Raman effect was much easier to measure than infrared absorption, Raman spectroscopy dominated the field of molecular structure determination until commercial infrared spectrometers became available in the 1940s [10]. 

The Raman effect relates to scattering of light. Raman found that illuminating a transparent substance such as water causes a small proportion of the light to emerge... 

The Raman effect is produced when the frequency of visible light is changed in the scattering process by the absorption or emission of energy produced by changes in molecular vibration and vibration-rotation quantum states. 

Figure 1.14 The spectral manifestation of the Raman effect, (a) The spectrum of the incident light, (b) The spectrum due to scattered (Rayleigh and Raman) light, (c) The Raman spectrum. The relative intensities of the incident, Rayleigh, and Raman hnes are quite different in real...

A dispersive phenomenon occurring when the wavelength of scattered electromagnetic radiation in the mid-infrared spectral region is shifted relative to that of the incident beam of exciting radiation. Spectral excitation is typically measured at a nonabsorbing wavelength, and the Raman effect occurs when the polarizabihty of a bond varies with the internuclear distance, as specified by the equation ... 

The Raman effect is analogous to fluorescence except that it is not wavelength dependent and does not require the molecule to have a chromophore. The energy shift in cm" due to inelastic scattering of laser radiation is measured rather than wavelength. The shifts measured correspond to the wavenumbers of the bands present in the middle-IR spectrum of the molecule. 

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