Raman effect

When light interacts with matter, most scattered photons retain the energy—and therefore the wavelength—of the incident light. In Raman scattering, a small fraction emerges with a different energy because energy has been exchanged with the material. The shifts correspond to changes in the vibrational or rotational-vibrational states available in molecules or, in solids, to vibrational modes of the material. The unshifted scattering is commonly called Rayleigh scattering; shifted scattering is Raman scattering. The shifted signal is weak compared with the unshifted light, which historically made it difficult to detect.

C. V. Raman’s interest in light scattering grew partly from questions about the blue appearance of the sea. Working in Calcutta with collaborators including K. S. Krishnan, he investigated light scattered by liquids. In February 1928, the group reported a new component of scattered light with a changed wavelength, distinguishing it from ordinary elastic scattering. Raman and Krishnan examined many liquids, and later measurements established the wavelength shifts more precisely. Russian physicists Grigory Landsberg and Leonid Mandelstam independently observed related scattering in quartz in 1928. Raman received the 1930 Nobel Prize in Physics for work on the scattering of light and discovery of the effect named after him.

A Raman spectrometer illuminates a sample, separates the scattered light by wavelength, and measures the small shifts from the incident light. The pattern of shifts forms a spectrum that can be compared with known spectral features to help identify chemical bonds, molecular structure or material phases. Because the measurement often requires little sample preparation and can be performed without destroying the sample, Raman methods are used in chemistry, materials science, biology and related fields. Suitability depends on the sample and measurement conditions; fluorescence or weak Raman signals can complicate analysis.

Raman spectroscopy complements techniques such as infrared spectroscopy because the two methods respond to molecular vibrations according to different physical selection rules. Raman measurements can be useful for aqueous samples and for characterizing materials, but the ordinary spontaneous Raman signal is intrinsically weak. Fluorescence from a sample can overwhelm the shifted light, and intense illumination can heat or alter sensitive specimens. Researchers use suitable excitation wavelengths, careful optical design, and enhanced or nonlinear Raman techniques to address these limitations; such methods have their own trade-offs.