Raman effects

In the Raman effect, incident radiation is inelastically scattered from a sample and shifted in frequency by the energy of its characteristic molecu- 

Sample Preparation Techniques in Analytical Chemistry, Edited by Somenath Mitra ISBN 0-471-32845-6 Copyright 2003 John Wiley Sons, Inc. 

As with optical spectroscopy, the Raman effect can be applied non-invasively in a wide range of environments. In contrast with infrared spectroscopy, Raman measurements do not require complicated sampling techniques. In addition, optical fiber probes can be used for bringing the laser light to the sample and transporting scattered light to the spectrometer, thus allowing remote detection of Raman spectra. 

The spatial and temporal resolution of Raman scattering are determined by the spot size and pulse length, respectively, of the exciting laser. Femto-liter volumes (ca. 1 pm3) can be observed using a confocal lens microscope, 

In addition to the Rayleigh scattering, another effect, Raman scattering, occurs. There is a small but finite probability that the incident radiation will transfer part of its energy to one of the vibrational or rotational modes of the molecule. As a result, the scattered radiation will have a frequency Vq —, where is the molecular frequency. Similarly, there is a 

Mirror to reflect scattered light into the detector 

The classical argument for the Raman effect can be expressed mathematically as follows. Let fi be the dipole moment induced by an electric field E then the polarizability, a, is defined by 

Thus the dipole moment oscillates with three distinct frequencies Vq with amplitude ocqE Vo and Vo + v with much smaller amplitudes, doc/dq)qo E. Therefore we observe a relatively intense beam at one frequency and two very weak beams at frequencies slightly above and below that of the intense one. 

Since the occurrence of the Raman effect depends on the change in polarizability as vibration occurs, the selection rules are different for the Raman effect than they are for the infrared spectrum. In particular, in molecules with a center of symmetry the totally symmetric vibration is Raman-active, but is forbidden in the infrared since it produces no change in dipole moment. Thus the homonuclear diatomic molecules, H2, O2, N2, show the Raman effect but do not absorb in the infrared. There is also a purely rotational Raman eff ect in these molecules. However, in this case the selection rule is A J = 2. Thus we have for the rotational Stokes lines 

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Raman effect When light of frequency Vo is scattered by molecules of a substance, which have a vibrational frequency of j, the scattered light when analysed spectroscopically has lines of frequency v, where... 

This spectrum is called a Raman spectrum and corresponds to the vibrational or rotational changes in the molecule. The selection rules for Raman activity are different from those for i.r. activity and the two types of spectroscopy are complementary in the study of molecular structure. Modern Raman spectrometers use lasers for excitation. In the resonance Raman effect excitation at a frequency corresponding to electronic absorption causes great enhancement of the Raman spectrum. 

B) THE MICROSCOPIC HYPERPOLARIZABILITY IN TERMS OF THE LINEAR POLARIZABILITY THE KRAMERS-HEISENBERG EQUATION AND PLACZEK LINEAR POLARIZABILITY THEORY OF THE RAMAN EFFECT... 

Long D A 1988 Early history of the Raman effect Int. Rev. Phys. Chem. 7 314-49... 

Furtak T E and Reyes J 1980 A critical analysis of the theoretical models for the giant Raman effect from adsorbed molecules Surf. Sc/. 93 351-82... 

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. 

Long, D. A. (2002) The Raman Effect, John Wiley, Chichester. 

In the context of discussion of the Raman effect, Equation (5.43) relates the oscillating electric field E of the incident radiation, the induced electric dipole fi and the polarizability a by... 

Vibrations allowed in the infrared are also allowed in the hyper Raman effect. 

Some vibrations which are both Raman and infrared inactive may be allowed in the hyper Raman effect. Indeed, the occasional appearance of such vibrations in Raman spectra in a condensed phase has sometimes been attributed to an effect involving the hyperpolarizability. 

Figure 9.21 Transitions in the stimulated Raman effect in benzene...

In the stimulated Raman effect it is only the vibration that gives the most intense Raman scattering that is involved this is the case for Vj in benzene. 

This Raman shifted radiation is obtained using the stimulated Raman effect (Section 9.3.2)... 

A similar calculation will show that the stimulated Raman effect applied to frequency tripled radiation from a Nd YAG laser, with a fundamental wavelength of 1064.8 nm, produces wavelengths of 299 nm, with H2, and 289 nm, with H2. 

Draw a diagram similar to that in Figure 9.21 to illustrate the stimulated Raman effect in FI2. Fligh-pressure FI2 is used to Raman shift radiation from a KrF laser. Calculate the two wavelengths of the shifted radiation which are closest to that of the KrF laser. 

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. 

Phonon Structure and Raman Effect of Single-Walled Carbon Nanotubes... 

In the following sections, we first show the phonon dispersion relation of CNTs, and then the calculated results for the Raman intensity of a CNT are shown as a function of the polarisation direction. We also show the Raman calculation for a finite length of CNT, which is relevant to the intermediate frequency region. The enhancement of the Raman intensity is observed as a function of laser frequency when the laser excitation frequency is close to a frequency of high optical absorption, and this effect is called the resonant Raman effect. The observed Raman spectra of SWCNTs show resonant-Raman effects [5, 8], which will be given in the last section. 

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. 

Before discussing specific examples of the application of Raman spectroscopy to studying adsorbate-adsorbent interactions, it will be necessary, at this juncture, to explain the nature of the Raman effect. 

The nature of the spectrum and the terminology of the Raman effect are summarized in Fig. 1. 

Koningstein, J. A., Introduction to the Theory of the Raman Effect, Chapter I. Reidel Publ., Dordrecht, Netherlands, 1972. 

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