RAMAN EFFECT. When a beam of monochromatic light passes through a transparent substance a certain amount of light is scattered from the path of the original beam which is of greater wave-length than the incident light. This effect was discovered by Raman in 1928, and is known by his name. It must be dis tinguished from the Tyndall effect and from ordinary fluorescence. In the Tyndall cone (see COLLOIDS) the scattered light is of exactly the same wave-length as the primary beam, supposing monochromatic light to be used : if mixed light is used each wave length is scattered without change of frequency, but the fact that the short wave-lengths are scattered in greater intensity than the longer wave-lengths leads to an apparent modification of colour. With fluorescence the light scattered in all directions is, except in a few exceptional cases, of wave-length greater than that of the primary beam (see FLUORESCENCE AND PHOSPHORESCENCE, where the exceptions to Stokes' law are discussed), but this wave length is characteristic of the particular fluorescent substance, and does not change with the wave-length of the incident light, so long as this is short enough to excite fluorescence at all. The Raman effect is distinguished by the fact that the frequency of the scattered light changes with that of the primary beam, the difference of frequency between the primary beam and the scat tered light being independent of the frequency of the primary beam. Thus, with the light from a mercury arc as the exciting radiation, each strong line is accompanied by a group of scattered lines, the frequency intervals from the primary line being the same within each group. Further, certain classes of chemical sub stances, such as different organic liquids each containing the same chemical group (e.g., the CH group), give in the scattered light groups of lines whose frequency intervals from the primary line are the same for all substances of the class, but these intervals vary from substance to substance in other cases. The Raman radiation resembles the Tyndall scattered light in that it is, in general, strongly polarised.
The effect is observed by illuminating pure dust-free liquids with an intense beam of light containing approximately mono chromatic radiations, e.g., the light from a mercury vapour lamp, and photographing the scattered light from a direction more or less at right angles to the original beam. The magnitude of the shift is of the order of i oo Angstrom units: it is not a question of a minute modification of frequency. The scattered light always contains the original frequency in comparatively great intensity, the modified lines often requiring a long exposure.
The interpretation of the effect is a matter of great theoretical importance. On the quantum theory of radiation (see QUANTUM THEORY) a certain quantum of energy hi, is to be attributed to the incident radiation, and a quantum of energy hi? to the scat tered radiation, h being Planck's constant and v and V the re spective frequencies. The difference h( v—il) must be absorbed by the molecule in some quantum change, and the order of magni tude of this difference corresponds to an infra-red frequency.
There is strong support for the view that the energy communicated to the molecule appears as energy of vibration of the nuclei of certain atoms in it, that is, the distance between the nuclei of these atoms varies periodically, the energy of the oscillations being governed by quantum conditions. Thus, associated with the strong mercury line 4,358A in the incident light, there is a scat tered line at 5,000A with organic compounds containing the CH group: the difference of wave-number is — = which corresponds to an infra-red line at 3.4A. Such a line is present in the infra-red spectra of these compounds, and is attributed to nuclear oscillation. (See BAND SPECTRUM.) Other scattered lines lead to other infra-red frequencies characteristic of the molecules in question. The Raman effect thus furnishes spectroscopy with a new and very powerful weapon for investigat ing the infra-red spectra of determined molecules without the very troublesome technique of infra-red measurements. In the case of complicated molecules the vibrations can be traced to simple groups within them, as exemplified by the CH group.
Besides the lines of lower frequency than the exciting light a few lines are found in the scattered light which have a higher frequency, and thus correspond to a contribution of energy by the molecule to the quantum of energy of the incident light. Such lines find a natural explanation in the existence of molecules which, as regards the nuclear vibrations, are in one of the excited quan tum states of higher energy : a quantum jump to a state of lower energy occurring in conjunction with the scattering process pro vides the necessary increment of energy to the scattered quant.
The Raman effect shows an interesting similarity to the Comp ton effect (q.v.). In the Compton effect the incident light-quant communicates part of its energy to a free, or loosely bound, elec tron, which energy appears as kinetic energy of this electron, the light quant of diminished energy being scattered as radiation of greater wave-length than that of the incident quant. The Comp ton effect is observed with hard X-rays, the magnitude to be anticipated for the effect with optical frequencies being too small for experimental observation. In the Raman effect the incident radiation sacrifices part of its energy in an interaction with matter, and reappears as a radiation of diminished frequency, just as in the Compton effect, but the energy lost as radiation appears as energy of molecular vibration instead of as kinetic energy of an electron.
BiBuocRapriv.—The discovery of the effect was first announced by C. V. Raman in the Indian Journal of Physics, ii., 387, 1928, and by Raman and K. S. Krishnan in Nature, CXXI., 501, 1928. Other papers on the subject are: Raman and Krishnan, Indian Joural of Physics, II., 399, 1928 ; Cabannes and Daure, Comptes Rendus de l'Academie des Sciences, CLXXXVI. 1533, 1928 ; G. Rogard, Comptes Rendus, CLXXXVI., 1107, 1928 ; P. Pringsheim, Die Naturwissen schaften, XVI., 597, 1928 ; R. W. Wood, Philosophical Magazine, VI.,