(Excerpts from the book, “K.S. Krishnan: his Life and Work”, by D.C.V. Mallik and S Chatterjee; Universities Press (2011))
Sitting under a deep blue sky, on the deck of the SS Narkunda, watching the blue opalescence of the Mediterranean Sea, Sir C.V. Raman was wondering about the mysterious qualities of light. It was September 1921 and Raman was returning from the Congress of Universities of the British Empire which was held in Oxford. Like a medical doctor with his stethoscope and first-aid kit, Raman too carried his experimental physicist’s little kit, consisting of pocket Nicol prisms, a small telescope and even a diffraction grating. The blue colour of the sea was attributed by many, including Lord Rayleigh, to the reflection of the blue of the sky from the water surface. Raman did not accept this. He had noticed that even when the sky was overcast, the blue of the sea was rather striking, a feature that could not be explained if the origin of it were just reflection. With his tools at hand, Raman performed some simple and illuminating experiments on board the ship by visually studying the polarisation of light. By a suitable orientation of the Nicol prism he was able to quench the reflection of the sky and found that the colour of the sea took on a deeper blue. He concluded the blue colour of the deep sea was a distinct phenomenon in itself. He wrote up his little paper, on board the ship, in the form of a letter to Nature and despatched it from the Bombay Harbour as he disembarked. This marked the beginning of a period of intense and extremely productive scientific activity at Raman’s laboratory at the Indian Association for the Cultivation of Science in Calcutta. The main thrust was the studying of the scattering of light in liquids and vapours. This eventually led to the discovery of the effect that was the crowning achievement of Indian science and remains so to this day.
Raman undertook a series of detailed experimental studies to investigate both these effects. His early collaborator in this work was K.R. Ramanathan who arrived in his laboratory in December 1921. An unexpected finding of the series of experiments was the presence of a trace of radiation in the scattered beam, in some of the liquids, that was of a different colour than the incident beam. Ramanathan observed that in water and ethyl alcohol, the depolarisation changed when filters of different colour were introduced in the path of the incident light. He attributed it to the ` presence of a trace of fluorescence’, since through a suitable arrangement of the filters, he could ascertain that the trace radiation was of a longer wavelength. But the trace radiation was very feeble and its polarisation properties were not investigated at the time. The weakness of the fluorescence bothered Raman. He did not rule out the possibility of the presence of fluorescent impurities in the liquid samples used.
In his first scientific paper in Phil. Mag., Sir K.S. Krishnan, the most well known student of Raman and later founder-director of the National Physical Laboratory, studied sixty five dust-free liquids to determine their scattering properties.In many of the liquids he detected the phenomenon of ` feeble fluorescence’ seen earlier by Ramanathan.
The experiments were performed in the first half of 1924. News had already reached Calcutta that C.V. Raman was being elected Fellow of the Royal Society in London that summer and there was great jubilation both at the University of Calcutta and at the Science Association. Soon after his election to the Fellowship, Raman was invited to attend a meeting of the British Association for the Advancement of Science (BAAS) in Toronto. At this meeting William Duane of Harvard University engaged in a lively debate with Arthur Compton, who had discovered the ` Compton Scattering’ in x-rays just the year before. Compton scattering was another phenomenon that clearly revealed the photon nature of the electromagnetic waves, albeit in a higher energy range than in the case of the photo-electric effect. Raman participated in the debate.
In December 1927, Arthur Compton received the Nobel Prize in physics. The news was brought to Raman by Krishnan who had just seen the announcement in an evening special. An eye-witness account, oft quoted in articles describing the discovery of the Raman Effect, says `Professor Raman beamed with delight and burst out in his characteristic fashion: “Excellent news … … very nice indeed. But look here Krishnan. If this is true of x-rays, it must be true of light too. I have always thought so. There must be an optical analogue to Compton Effect. We must pursue it and we are on the right lines. It must and shall be found.”
During the same time, Raman had asked one of his students, Venkateswaran, to study light scattering in ‘highly viscous organic liquids which were capable of passing over into the glassy state.’ In due course, when Venkateswaran experimented with a purified sample of glycerine using sunlight, he found that the scattered beam was an intense green instead of the usual blue. The scattering process was without doubt non-coherent and since the radiation was quite strong, it could be analysed further and there was indication that it was polarised. As Raman said in his Nobel lecture:
The phenomenon appeared to be similar to that discovered by Ramanathan in water and the alcohols, but of much greater intensity, and therefore, more easily studied. No time was lost in following up the matter.
Raman needed Krishnan to do the ` following up’. The key issue was the investigation of the polarisation of the scattered radiation. K S Krishnan, who was meticulous in his work maintained a diary describing the day to day events at the laboratory during this period. The diary entry on the 5th of February, 1928 stated:
Recently Professor has been studying with Mr Venkateswaran the fluorescence exhibited by many of the aromatic liquids in the near ultraviolet region present in sunlight and the fluorescence of some of the liquids was found to be polarised. However, in view of the fact that the fluorescence of anthracene vapour does not show any polarisation Professor asked me to verify again his observations in some of the liquids.
The experimental set-up had been perfected over the years and once the goal was set, it took Krishnan only a few days to accumulate data to elucidate the true character of the `feeble fluorescence’, which had remained a mystery since 1923-4. The diary entry on the 7th of February gave the first definitive clue:
Incidentally discovered that all pure liquids show a fairly intense fluorescence in the visible region, and what is much more interesting all of them are strongly polarised; the polarisation being the greater for the aliphatics than for the aromatics. In fact the polarisation of the fluorescent light seems in general to run parallel with the polarisation of the scattered light, i.e. the polarisation of the fluorescent light is greater the smaller the optical anisotropy of the molecule.
When I told Professor about the results he wouldn’t believe that all liquids can show polarised fluorescence and that in the visible region. When he came into the room, I had a bulb of pentane in the tank blue with violet filter in the path of the incident light, and when he observed the track with a combination of green & yellow filters he remarked “ you don’t mean to suggest, Krisnayengar all that is fluorescence?!” However when he transferred the green yellow combination also to the path of the incident light he couldn’t detect a trace of the track. He was very much excited and repeated several times it was an amazing result. One after another the whole series of liquids were examined and every one of them showed the phenomenon without exception. He wondered how we missed discovering all that five years ago…
After meals at night Venkateswaran and myself were chatting together in our room when Professor suddenly came to the house (at about 9 P.M.) and called for me. When we went down we found he was very much excited and had come to tell me that what we had observed this morning must be the Kramers-Heisenberg effect we had been looking for all these days. We therefore agreed to call the effect modified scattering rather than fluorescence.
The rapid stride of events of the following weeks is described in reasonable detail in Krishnan’s diary. The first paper announcing the discovery was jointly authored by Raman and Krishnan in the form of a letter to Nature titled ‘A new type of secondary radiation’ and was despatched on February 16, 1928 to Nature. Some sixty odd liquids and a few vapours had been examined exactly the way Krishnan described in his diary and ‘every one of them showed the effect in greater or less degree’. As they stated in the paper:
That the effect is a true scattering and not a fluorescence is indicated in the first place by its feebleness in comparison with the ordinary scattering, and secondly by its polarisation, which is in many cases quite strong and comparable with the polarisation of the ordinary scattering.
The paper was hardly fifty one lines long and carried no spectra although it stated that ‘ Spectroscopic confirmation is also available’. On February 28, Raman and Krishnan decided to further restrict the range of wavelengths of the incident beam of sunlight by combining a uranium glass with the usual blue-violet filter and when they observed the scattered beam with a direct vision spectroscope, they found, to their ‘great surprise, that the modified scattering was separated from the scattering corresponding to the incident light by a dark region.’ This encouraged them to use a mercury arc with its line spectrum as a monochromatic source. By a suitable filter they eliminated all radiation red-ward of the prominent 4358 Å line and watched the scattered radiation through a spectroscope. Unfortunately, Krishnan’s diary abruptly ends and no eye-witness account has survived of the squeal of delight that must have emanated from Raman’s lips on seeing the faint additional lines in the scattered beam. It is not hard to imagine the thrill both the experimenters had felt at the moment. The Raman Effect was discovered and the 28th of February is now celebrated annually as the National Science Day. Krishnan was asked to photograph the spectrum of the scattered beam. This needed some ingenuity and enormous patience, since the weakness of the ‘modified’ lines required long exposures to be taken. The first liquids examined were benzene, toluene and pentane and in each the presence of the `modified’ lines was seen. Next day, the news of the discovery was released to the Associated Press.
The next few months were a busier than ever time in the Science Association and Raman and Krishnan worked at a feverish pace to reap the rich harvest of knowledge the new field was yielding. Elsewhere in the world too spectroscopists plunged into exploring all aspects of the new phenomenon with the instruments at their disposal. R W Wood working in the physical laboratory of the Johns Hopkins University in the United States confirmed the results found by Raman and Krishnan.
Raman scattering is the result of a double transition involving three stationary levels and results in an internal excitation or a de-excitation of the scattering molecule. Since the final transition is mediated via a virtual second level, the selection rules governing the phenomenon are somewhat different from the ones that govern the direct transitions between the vibrational and rotational levels of a molecule. This is especially true where transitions between the rotational levels are involved. Thus the infra-red absorption spectra of molecules are complementary in character to the Raman spectra. In simplest terms, the scattering event may be visualised as follows: a light quantum of energy hν collides with a molecule in a state n where the photon gives up a part of its energy to the molecule which makes a transition to an intermediate or virtual state r followed by a decay to a state m, with an energy higher than the energy in the state n. The scattered photon then emerges with an energy hν́, which differs from that of the incident photon by just the excitation amount of the state m, i.e.,
hν – hν́ = Em – En = ΔE,
and the experimenter sees a Raman line at a frequency ν́, lower than the frequency of the Rayleigh line which appears at ν. If the state n were already an excited one, and the photon gained energy in the scattering process leaving the molecule in a lower state of excitation, an anti-Stokes line would be produced. In general, ΔE may take a number of different values and the Raman shifts directly give the energy differences of the system under study.
Since an anti-Stokes line is emitted in a scattering event with a molecule in an excited state and since at ordinary temperatures the population in such states is degraded by a Boltzmann factor, an anti-Stokes line is always much weaker than its Stokes counterpart and hence more difficult to detect. Krishnan had to expose the spectrum of benzene for several hours before the feeble traces of the anti-Stokes lines appeared in his record. It was an experimental marvel given the rather moderate nature of the set-up in the laboratory of the Science Association.