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 alternation in phase is known as Raman effect. 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.
Discovery: Because of its low instensity 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. Prompted by A.H. Compton’s observation of frequency changes in X-rays scattered by electrons (Compton effect), the Indian Physicists C. Raman and K.S. Krishnan examined sunlight scattered by a number of liquids. With the help of the complementary fitters, they found that there were frequencies in the scattered light that were frequencies in the scatterd light that were lower than the frequencies in the filtered sunlight.They then showed, by using light of a single frequency from a mercury are, that the new frequencies in the scattered radiation were characteristic of the scattering medium. Within a few months of Raman and Krishnan’s first announcement of their discovery, the Soviet Physicists G. Landsberg and L. Mandelstam communicated their independent discovery of the existence of the effect in crystals. In Russian literature the phenomenon is reffered to as combination scattering, and not Raman effect.
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. A beam of laser radiation is intense, polarized, and coherent, it can be made monochromatic, small in diameter, nearly ideal for the production of the Raman effect, and other kinds of sources are seldom employed. Many different wavelengths in the visible spectrum and adjacent regions are available. The argon- ion and krypton-ion lasers are most commonly used, since they have high continuous wave power(1-10 Watts), but tunable dye lasers are also often employed in excitation of resonance Raman scattering.
Raman Spectroscopy: 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. And, because of the laser beam’s small diameter and high collimation, it can be used to excite the Raman effect.
Theory: The mechanism of the Raman effect can be envisaged either by the corpuscular picture of light or from the point of view of the wave theory. Both pictures merge in the basic quantum theory or radiation. The corpuscular model of light scattering envisages light quanta or photons as particles which have linear and angular momentum. On passing through a material medium, these particles collide with atoms or molecules. If the collision is elastic, the photons bounce off the molecules with unchanged energy, E and momentum, and hence with unchanged frequency V. Such a process gives rises to Rayleigh scattering. If the collision is inelastic, the photons may gain energy from, or lose it to, the molecules. A change ∆v=∆A/h. Such inelastic collisions are rare compared to the elastic ones, and the Raman effect is correspondingly much weaker than Rayleigh scattering.
In the wave picture of the effect, the electromagnetic waves which constitute the incoming monochromatic radiation sweep through the material medium. Since the atoms and molecules composing the medium are made up of negatively charged electrons and positively charged nuclei, the electric field of the light waves sets the electron to oscillating, chiefly with the frequency of the incoming radiation. The oscillating electrons recreate the alternating electric field of the incoming light, thus passing the light wave along through the medium. This process is analogous to the elastic collisions which are given by the corpuscular picture.
The ability of the electrons and nuclei in a molecule to be displaced by an electric field is called the molecular polarizability Þ. It is not a simple property of the molecule, but depends in a complicated way on the frequency of the electric field, on the orientation of the molecular, and on the internal motions of the nuclei and electrons. Thus, the molecular polarizability. Þ varies periodically with the molecular rotation and vibration, and thereby the effect of a light wave on the electrons and nuclei of a molecule can be changed.
When a chromatic light wave sweeps through a transparent medium containing rotating and vibrating molecules, most of the wave is recreated unchanged by the oscillating electrons, but because of the periodic changes produced in Þ by rotation and vibration, new frequencies are added to the light wave, whose values are determined by the rotational and vibrational energies of the molecules, is analogous to the corpuscular model. For the wave picture of the Raman effect, the quantity Þ is the basic quantity. The intensity of the Raman effect depends on the magnitude of the changes produced in Þ by molecular rotation and vibration, and the number and values of new frequencies depend on the variation of Þ with the frequencies of rotation and vibration.
The temperature of the scattering molecules is an additional factor which affects the intensity of Raman frequencies higher than the exciting frequency. If the molecules do not have any available vibrational or rotational energy, that is, if they are at the absolute zero of temperature, there is no possibility of inelastic collisions in which energy is transferred from a molecule to a photon.
Applications: 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 characteristics 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. The complex structures of biologically important molecules, for example, are the subjects of current spectroscopic research. Both normal and resonance Raman spectroscopy are valuable techniques in molecular biology. Raman spectra also provide information for solid-state physicists, particularly with respect to lattice dynamics but also concerning the electronic structure of solids.
