Raman Effect


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Raman effect

(rä`mən), appearance of additional lines in the spectrumspectrum,
arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph).
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 of monochromatic light that has been scattered by a transparent material medium. The effect was discovered by C. V. Raman in 1928. The energy and thus the frequency and wavelength of the scattered light is changed as the light either imparts rotational or vibrational energy to the scattering molecules or takes energy away. The line spectrum of the scattered light will have one prominent line corresponding to the original wavelength of the incident radiation, plus additional lines to each side of it corresponding to the shorter or longer wavelengths of the altered portion of the light. This Raman spectrum is characteristic of the transmitting substance. Raman spectrometry is a useful technique in physical and chemical research, particularly for the characterization of materials.

Raman effect

A phenomenon observed in the scattering of light as it passes through a material medium, whereby the light suffers a change in frequency and a random alteration in phase. Raman scattering differs in both these respects from Rayleigh and Tyndall scattering, in which the scattered light has the same frequency as the unscattered and bears a definite phase relation to it. The intensity of normal Raman scattering is roughly one-thousandth that of Rayleigh scattering in liquids and smaller still in gases. See Scattering of electromagnetic radiation

Because of its low intensity, the Raman effect was not discovered until 1928, although the scattering of light by transparent solids, liquids, and gases had been investigated for many years before. The development of the laser has led to a resurgence of interest in the Raman effect and to the discovery of a number of related phenomena. See Laser

When the exciting radiation falls within the frequency range of a molecule's absorption band in the visible or ultraviolet spectrum, the radiation may be scattered by two different processes, resonance fluorescence or the resonance Raman effect. Both these processes give much more intense scattering than the normal nonresonant Raman effect. The absolute frequencies of the resonance Raman effect shift by exactly the amount of any shift in the exciting frequency, just as do those of the normal Raman effect. Thus the main characteristic of the resonance as compared to the normal Raman effect is its intensity, which may be greater by two or three orders of magnitude. See Fluorescence

Raman scattering is analyzed by spectroscopic means. The collection of new frequencies in the spectrum of monochromatic radiation scattered by a substance is characteristic of the substance and is called its Raman spectrum. Although the Raman effect can be made to occur in the scattering of radiation by atoms, it is of greatest interest in the spectroscopy of molecules and crystals. In a typical experiment monochromatic radiation from a laser impinges on the sample in an appropriate transparent cell. Raman scattering is approximately uniform in all directions and is usually studied at right angles. In this way the intense radiation of the laser beam interferes least with the observation of the weak scattered light.

Raman spectroscopy is of considerable value in determining molecular structure and in chemical analysis. Molecular rotational and vibrational frequencies can be determined directly, and from these frequencies it is sometimes possible to evaluate the molecular geometry, or at least to find the molecular symmetry. Even when a precise determination of structure is not possible, much can often be said about the arrangement of atoms in a molecule from empirical information about the characteristic Raman frequencies of groups of atoms. This kind of information is closely similar to that provided by infrared spectroscopy; in fact, Raman and infrared spectra often provide complementary data about molecular structure. Raman spectra also provide information for solid-state physicists, particularly with respect to lattice dynamics but also concerning the electronic structures of solids. See Infrared spectroscopy, Lattice vibrations, Molecular structure and spectra

Raman Effect

 

the scattering of light by matter, accompanied by a noticeable change in the frequency of the scattered light. If a source emits a line spectrum, the Raman effect produces additional lines, whose number and location are closely related to the molecular structure of the substance, in the spectrum of the scattered light.

The Raman effect was discovered in 1928 by the Soviet physicists G. S. Landsberg and L. I. Mandel’shtam during studies of the scattering of light in crystals and simultaneously by the Indian physicists C. V. Raman and K. S. Krishnan during studies of the scattering of light in liquids. In the Raman effect the transformation of the primary light flux usually is accompanied by transition of the scattering molecules to other vibrational and rotational energy levels in such a way that the frequencies of the new lines in the scattering spectrum are combinations of the frequency of the incident light and the frequencies of the vibrational and rotational transitions of the scattering molecules (hence the Russian name, kombinatsionnoe rasseianie sveta, or “combination light scattering”).

To observe Raman-effect spectra, an intense light beam must be concentrated on the object under study. A mercury lamp— or, since the 1960’s, a laser beam—is most often used as the source of the exciting light. The scattered light is focused and strikes a spectograph, where the Raman-effect spectrum is recorded by photographic or photoelectric methods.

The Raman effect is most often associated with a change in the oscillatory states of molecules. Such a Raman-effect spectrum consists of a system of companions that lie symmetrically about an exciting line with frequency v (Figure 1). A companion with frequency v+ v, (a violet, or anti-Stokes, companion) corresponds to every companion with frequency vvi (a red, or Stokes, companion). Here vi is one of the natural oscillation frequencies of the molecule. Thus the frequencies of the natural (or normal) oscillations of a molecule, which are manifested in the Raman-effect spectrum, may be determined by measuring the frequency of Raman-effect lines. Similar mechanisms also exist for a rotational Raman-effect spectrum. In this case the frequencies of the lines are determined by the rotational transitions of the molecules. In the simplest case a rotational Raman-effect spectrum is a sequence of nearly equidistant, symmetrically situated lines whose frequencies are combinations of the rotational frequencies of the molecules and the frequency of the exciting light.

Figure 1. Diagram of Stokes lines (with frequencies v — v1v — v2 and v — v3) and anti-Stokes lines (v + v1,v + v2, and v + v3) during Raman scattering of light with frequency v

According to quantum theory, the process of the Raman effect consists of two interconnected events, the absorption of a primary photon with energy hv (where h is Planck’s constant) and the emission of a photon with energy hv' (where v’ = v ± vi), which take place as a result of the interaction of the molecule’s electrons with the field of the incident light wave. Under the action of a quantum with energy hv through the compound state, a molecule in an unexcited state passes into a state with oscillatory energy hvi, emitting a quantum h(v — vi). This process leads to the appearance of a Stokes line with frequency v — vi in the scattered light (Figure 2,a). If the photon is absorbed by a system in which oscillations already have been excited, then after scattering it may pass into a zero state; here the energy of the scattered photon exceeds the energy of the absorbed photon. This process leads to the appearance of an anti-Stokes line with frequency v + vi (Figure 2,b).

The probability w of the Raman effect (and consequently the intensity of thn Raman-effect lines) depends on the intensity of the exciting radiation I0 and the scattered radiation I:w =aI0(b + I), where a and b are some constants; when the Raman effect is excited by ordinary light sources (such as a mercury lamp), the second term is small and may be disregarded. The intensity of the Raman-effect lines is extremely low in most cases, and at ordinary temperatures the intensity of anti-Stokes lines Ia generally is much less than the intensity of the Stokes lines Is. Since the probability of scattering is proportional to the number of scattering molecules, the ratio Ia/Is is defined by the

Figure 2. (a) Stokes transitions, (b) anti-Stokes transitions during Raman scattering: (G) ground level, (hvi) vibrational level, (hvi) intermediate electron level of the molecule

ratio of the populations of the ground and excited levels. At ordinary temperatures the population of the excited levels is not great, and consequently the intensity of the anti-Stokes component is low. The population rises with increasing temperature, leading to an increase in the intensity of the anti-Stokes lines. The intensity of Raman-effect lines / depends on the frequency v of the exciting light: at great distances (on the frequency scale) from the molecules’ region of electron absorption, I ~ v4; as the electron absorption band is approached, a more rapid increase in their intensity is observed. In some cases, when the concentration of matter is low, it is possible to observe a resonance Raman effect, in which the frequency of the exciting light enters the region of the substance’s absorption band. When the Raman effect is excited by high-powered lasers, its probability increases and a stimulated Raman effect, whose intensity is of the same order as that of the exciting light, arises.

Raman-effect lines are polarized to a greater or lesser extent. Here various companions of a given exciting line have different degrees of polarization, but the nature of the polarization of Stokes and anti-Stokes companions is always identical.

The Raman effect, like infrared spectroscopy, is an effective method for studying the structure of molecules and their interaction with the surrounding medium. It is significant that the Raman-effect spectrum and the infrared absorption spectrum do not duplicate each other, since they are defined by different selection rules. The symmetry of normal oscillations—and, consequently, the symmetry of the molecule as a whole—may be assessed by comparing the frequencies of the lines in the Raman-effect spectrum and the infrared spectrum of a given chemical compound. In this manner, a real model corresponding to the mechanisms in the observed spectra may be chosen from among several proposed models of a molecule. In many cases the frequencies and other parameters of Raman-effect lines are retained upon transition from one compound to another that has the same structural element. This so-called characteristic nature of the parameters of Raman-effect lines underlies the structural analysis of molecules of unknown structure.

In crystals the Raman effect has certain distinctive features. The oscillations of atoms in a crystal may be identified with a phonon gas, whereas the Raman effect in crystals may be viewed as scattering by phonons. Other quasiparticles of a crystal (such as polarons and magnons) are also studied using methods involving the Raman effect.

The Raman-effect spectra of every compound are so specific that they may serve to identify the compound and to detect it in mixtures. Qualitative and quantitative analysis based on Raman-

Figure 3. Diagrams of apparatus for observing the Raman effect by using lasers: (a) transparent object (liquid or crystal), (b) powdered substance (transillumination method), (c) reflection method; (K1) and (K1) lenses, (O) object, (A) spectrograph aperture, (S) screen to eliminate exciting radiation

effect spectra are used widely in analytic practice, especially in the analysis of hydrocarbon mixtures.

As a result of the use of lasers as sources of exciting light (Figure 3), the range of objects that may be studied by methods involving the Raman effect has broadened greatly, and it has become possible to study in greater detail gases, powders, and tinted substances, for example, semiconductor materials. In addition, the use of lasers has sharply reduced the required quantity of the substance under study.

REFERENCES

Landsberg, G. S. Izbr. trudy. Moscow, 1958. Pages 101–70.
Mandel’shtam, L. I. Poln. sobr. trudov, vol. 1. Moscow, 1947. Pages 293 and 305.
Raman, C. V., and K. S. Krishnan. “A New Type of Secondary Radiation.” Nature, 1928, vol. 121, no. 3048, p. 501.
Sushchinskii, M. M. Spektry kombinatsionnogo rasseianiia molekul i kristallov. Moscow, 1969.
Light Scattering Spectra of Solids. Edited by G. B. Wright. Berlin, 1969.
Landsberg, G. S., P. A. Bazhulin, and M. M. Sushchinskii. Osnovnye parametry spektrov kombinatsionnogo rasseianiia uglevodorodov. Moscow, 1956.
Brandmüller, J., and G. Moser. Vvedenie v spektroskopiiu kombinatsionnogo rasseianiia sveta. Moscow, 1964. (Translated from German.)
Bobovich, la. S. “Poslednie dostizheniia v spektroskopii spontannogo kombinatsionnogo rasseianiia sveta.” Uspekhi fizicheskikh nauk, 1969, vol. 97, issue 1, p. 37.

M. M. SUSHCHINSKII


Raman Effect

 

the scattering of light by matter, accompanied by a change in the frequency of the scattered light. The Raman effect was discovered in 1928 by G. S. Landsberg and L. I. Mandel’shtam in crystals and simultaneously by the Indian physicists C. V. Raman and K. S. Krishnan in liquids. The term “Raman effect” is widely encountered in foreign literature.

Raman effect

[′räm·ən i‚fekt]
(optics)
A phenomenon observed in the scattering of light as it passes through a transparent medium; the light undergoes a change in frequency and a random alteration in phase due to a change in rotational or vibrational energy of the scattering molecules. Also known as Raman scattering.
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