Sir C.V.Raman and Raman Spectroscopy (1888-1970)
Sir C.V.Raman won the Nobel Prize in 1930 for discovering a new type of radiation when a material is excited by an external light source. This radiation was nothing but a signature of the molecular bonds holding the molecules together which was unique for every material. A look into the background of Sir C.V.Ramans life and a little description about the famous Raman Effect.
Because of ill health Raman was unable to pursue further studies at any of the universities in Great Britain (He was disqualified medically by the Civil Surgeons of Madras, who said that the rigors of English Climate would kill him) and because at that time there were no opportunities or incentives in India for a scientific career, he entered the Indian Civil Service, attaining first place on the competitive examination for a position in the Indian finance Department (The Indian Audit and Account Service was the only Government department that did not require a training period in Great Britain) . In June 1907 he was posted at Calcutta as Assistant Accountant General, and that same year he married 13-year old Lokasundari Ammal, an artist who shared his interest in musical instruments, by whom he had two sons, Chandrasekhara and Radhakrishnan. He served the Indian Finance Department in posts of increasing responsibility for a decade, much as Einstein worked as an examiner in the Swiss Patent Office. Like Einstein, he continued to carry out independent research working nights and weekends, mostly at the laboratory of the Indian Association for the Cultivation of Science at Calcutta. He worked primarily on vibrations and sound (Theoretical and experimental investigations of the oscillations of strings) and on the theory of musical instruments, especially that of violins and Indian drums.
During this period Raman published 30 papers in Nature, the Philosophical magazine, and the Physical review, which led to his being offered in 1917 the newly endowed Palit professorship of Physics at the University of Calcutta. Despite a considerable financial sacrifice, he resigned his better paying government post to accept this chair, which he held for 16 years, during which time he continued his work on acoustics and optics and made Calcutta a center for scientific research.
In the summer of 1921 Raman represented his University at the British Empire Universities Congress at Oxford and lectured on the theory of stringed instruments before the Royal Society of London. Returning home by the way of the Mediterranean Sea, he was fascinated by its opalescent, deep blue color and attempted to discover the cause. Rejecting Lord Rayleigh’s explanation that it was caused by the reflection from the sky, in 1922 Raman showed that the scattering of light by water molecules could account for the color of the sea exactly as the scattering of light by air molecules accounted for the color of the sky.
Despite his increasing preoccupation with optics, Raman also continued his research on acoustics, which resulted in his election to fellowship in the Royal Society in 1924. He worked on the excitation of strings vibrations; motion of the bowed point; the effect of the bridge in coupling the motion of the string to the body of the violin; vibration phenomena of the piano, veena and sitar; and the harmonic overtones of Indian drums. He also traveled extensively, visiting and representing India in Canada, the United States (At Nobel laureate Robert A.Millikan’s invitation he spent five months at the California Institute of technology in 1924), the USSR, Germany, Switzerland and Italy. In India he was active in organizing learned societies and journals, e.g. the Indian Science Congress (1924), The Indian Academy of Sciences (1934) and its Proceedings (In which much of his work was published) and the Indian Journal of Physics (which he founded and of which he became editor in 1926).
In the field of optics his primary concern, Raman studied light scattering by various substances, especially liquids. In April 1923 his associate K.R.Ramanathan observed a weak secondary radiation, shifted in wavelength along with normally scattered light, which was attributed to ‘fluorescence’. In January 1928, S.Venkateswaran next noticed that highly purified glycerin does not appear blue under sunlight but instead radiates a strongly polarized, brilliant, green light, they reported in Nature (March 31,1928) ‘ a new type of secondary radiation’ from the scattering of focused beams of sunlight in both carefully purified liquid and dust free air.
Raman further refined his experiment by using a mercury arc as the light source on February 28, 1928, and on March 16 he reported his results to the Indian Science Association at Bangalore in the Southern Indian state of Karnataka (‘A New Radiation’, Indian Journal of Physics, 2,387 (1928)). This secondary radiation showed several lines shifted toward longer wavelengths (The Shifts were characteristics of the substances used) and indicated the absorption of energy by the scattering molecule, an effect predicted in 1923 by the Austrian Physicist Adolf Smekal (1895-1959) and observed independently, but in less detail, several months after Raman and Krishnan’s discovery by the Soviet Physicists Grigorii Samuilovich Landsberg (1890-1957) and Leonid Isaakovich Mandelshtam (1879-1944). For his discovery, known as the Raman Effect, which Ernest Rutherford described as ‘among the best three or four discoveries in experimental physics of the decade’, Raman was awarded the Hughes Medal of the Royal Society of London (1930), the Matteucci Medal of the Societa Italiana delle Scienze (1928) was knighted by King George V of great Britain (1929) and received honorary degrees from numerous universities. In 1930 he received that ne plus ultra of science, the Nobel prize (in Physics) ‘ for his work on the scattering of light and for the discovery of the effect named after him’. He was the first Asian to receive a Nobel Prize in science. Raman continued his work on the Raman effect through the 1930’s, as did many others; almost 2,000 articles on it were published during the dozen years following its discovery.
Beginning at 1930, Raman divided his time between training future leaders of science and crystallographic studies that he thought ‘will ultimately have repercussions in the whole scientific world’. Believing that two and possibly four types of diamonds based on tetrahedral and octahedral symmetry must exist; he used a large portion of his $40,000 Nobel Prize to but diamonds. He studied their Raman spectra, fluorescence, absorption spectra, magnetic susceptibility, specific heat, X-ray diffraction patterns, and infra red spectra, and he demonstrated that the luminescence of a diamond excited by ultraviolet light is a characteristic of the diamond itself and is not due to impurities or defects, as was previously believed. He also investigated optical effects in minerals (labradorite, pearly felspar, and agate) and gems such as opal and pearls.
In 1933, Raman moved to Bangalore to become Director (1933-1937) of the Indian Institute of Science and Head of its Department of Physics (1933-1948). Here he pursued experimental and theoretical work on the diffraction of light by acoustic waves of ultrasonic and hypersonic frequencies, Brillouin scattering in liquids, light scattering in colloids, and the effects produced by X-rays on Infrared vibrations in crystals exposed to ordinary light. In 1948 Raman became the first Director of the Raman Research Institute built on land in Hebbal, a suburb of Bangalore, give by the Mysore government to the Indian Academy of Sciences. That same year he was appointed the first National Professor by the Government of a newly independent India.
Among topics investigated by Raman during his later years were colors and their perception, the spectroscopic study of flowers (the first to do this), the physiology of human color vision (he developed a new theory.), and electrical and magnetic anisotropy. A proud man of great authority, Raman was often arrogant and contemptuous of others, yet he was generous in encouraging his students. He often represented India at international meetings, was fluent in English slang, and considered himself a humorist. Educated entirely in India, he carried out pioneering scientific research when few Indians made science a career and when the Indian scientific community was small and relatively isolated. For his championing of Indian science, he was regarded, along with Rabindranath Tagore, Mahatma Gandhi, and Jawaharlal Nehru, as one of the heroes of the Indian political and cultural renaissance. He died on November 21, 1970 in Bangalore and was cremated in his beloved rose garden.
Raman Effect:-
The Raman effects, a scattering of a portion of monochromatic light when it is passed through a transparent substance, is the counterpart for visible light of the Compton effect for the X-rays. The American experimental physicist Robert Williams Wood considered it ‘one of the most convincing proofs of the quantum theory of light’. In addition to the light of the original frequency, the spectrum of the scattered light contains weaker lines (Raman lines) differing from the original frequency by constant amounts and due to the grain or loss of their energy experienced by the photons because of their interaction with the vibrating molecules of the substance through which they pass.
Since no two compounds have the same Raman Spectrum and since the intensity of a Raman line of a substance is proportional to its concentration, Raman spectroscopy can be used in qualitative and quantitative analysis. It is also employed in the elucidation of the structures of molecules, the study of the interactions between molecules and the calculation of thermodynamic properties.
Much information about a molecule can be deduced from its Raman spectrum. For example, for a diatomic molecule the effective moment of inertia in the lowest vibrational energy state and from this the effective inter-nuclear distance for this state can be obtained. Because homo-nuclear molecules such as H2 and N2 have no permanent dipole moment, they yield no pure rotational or vibrational infrared spectrum and therefore can be studied only by their Raman effect (although electronic spectra can yield rotational spectra). From an analysis of the vibrational-rotational bands there can be obtained the classical vibrational frequency of the diatomic molecule, the moments of inertia and inter-nuclear distances for higher vibrational states, the force constant, the dissociation energies, the zero point energy, the potential energy curve as a function of the inter-nuclear distance, and the rotational and vibrational energies needed for thermodynamic calculations.
Raman spectral data can also be used to detect interactions between molecules. For example, if hydrogen bonding is present, Raman displacements corresponding to C single bond H and C double bond O vibrations appear at values much lower than their normal values. The weakness of the scattered radiation have been the greatest deterrent to the widespread adoption of Raman spectroscopy as an analytical technique. Since the more intense the primary radiation source, the greater the intensity of the Raman spectra, the advent of lasers transformed this method of largely academic interest into a highly practical, powerful tool with a number of commercially available instruments and installations in many industrial laboratories. If Raman were alive today, he would undoubtedly be amazed - and –pleased by the multifaceted uses now made of his ‘new’ radiation.
