Of scattered light & more

remarkable

Chandrasekhara Venkata Raman or C V Raman was one of the greatest minds India ever saw. Widely known for his experiments on scattering of light, Prof Raman was the first Indian Nobel Laureate in science. Come February 28, the nation will celebrate the day as National Science Day, in commemoration of the discovery of the Raman Effect.

Born in 1888, Prof Raman began his career as a civil servant in Kolkata in 1907. Since he was passionate about physics, he took to doing experimental work in the Indian Association for Cultivation of Science. His early work was in acoustics of musical instruments. Later, he became interested in the structure of crystals, especially diamonds. He took up professorship in Calcutta University in 1917, where he worked for 15 years. Later, he served as the director of the Indian Institute of Science in Bengaluru from 1934 to 1948 and of the Raman Research Institute from 1949 until his death in 1970.

C V Raman is widely known for his work on scattering of light. His research in this particular field earned him a Nobel Prize (Physics) in 1930. The Raman Effect was honoured with the rank of being a National Historic Chemical Landmark by the American Chemical Society on December 15, 1998 and also got designated as an International Historic Chemical Landmark in the year 2013. Raman Effect is the change in wavelength of light that occurs when a light beam is deflected by molecules. When a beam of light traverses a transparent sample of a chemical compound, a small fraction of it emerges in directions other than that of the incident beam. Most of this scattered light is of unchanged wavelength; a small part, however, has wavelengths different from that of the incident light.

A matter of light

In fact, this effect can be easily understood if the incident light is treated as a source of photons. Most of the encounters of the particles with the target are what is called elastic scattering, where there is no change in energy. However, in a few encounters, the energy of the photon is changed by either giving energy or taking energy from the molecule. Thus, the scattered light will have a frequency (and colour) different than that of the incident light. Since the phenomenon can be understood only with the photonic aspect of light, this effect was also seen as one of the proofs for the quantum theory. Two years later, along with S Bhagavantham, a fellow scientist, Prof Raman was able to show that “the light quantum possesses an intrinsic spin equal to one Bohr unit of angular momentum,” which further confirmed the quantum nature of light.

The unique spectrum of Raman scattered light for any particular substance serves as a ‘fingerprint’ that could be used for qualitative analysis of solids, liquids gases and even a mixture of materials. Further, the intensity of the spectral lines is related to the amount of the substance. While generally, only one part in a thousand of the total intensity of incident light is Rayleigh scattering, this value drops to one part in a million for Raman scattering.  

Since 1980’s, with improved instrumentation, many new applications of Raman Effect have been found. Its ability to detect even small amounts of chemical and biological molecules has been helpful in the treatment of cancer, malaria, HIV and other illnesses. It now finds itself useful in scenarios like analysing nuclear waste material, detecting trace amounts of molecules in fraudulent paintings, chemical weapons, identifying dangerous substances such as improvised explosive devices at airports and so on.

Raman Effect is also playing an important role in astronomy. The feasibility of using the Raman spectrum to investigate the physical structure of outer planet atmospheres has been examined. Raman scattering makes a vital contribution to spectra because of very large amounts of hydrogen molecules in planets’ atmospheres. The spectra of Uranus and Neptune in the UV and visual range have been detected and these observations give information on the amount of hydrocarbons in the atmosphere. In 2004, Journal of Raman Spectroscopy published a paper titled Raman spectroscopy, breaking terrestrial barriers. It propagated that Raman spectroscopy can provide highly specific chemical fingerprints of inorganic and organic materials and is therefore, expected to play a significant role in interplanetary missions, especially in the search for life elsewhere in our solar system. 

The simplicity in Raman spectra and the non-ambiguity for phase identification are the keys for its application in planetary explorations. Future planetary missions of NASA and ESA to Europa and Mars  are all expected to carry Raman spectrometers. A Raman spectrometer is also being miniaturised for the ExoMars Rover and  is expected to identify organic compounds that could be related to signatures of life like cyanobacteria, chlorophyll, or amino acids. NASA’s mission to Europa, an important satellite of Jupiter, will try to analyse the surface environment. Raman spectra will be used to measure the key habitability parameters such as temperature, pH etc there. A study titled Stand-off Raman spectroscopic detection of minerals on planetary surfaces, conducted at the Hawaii Institute of Geophysics and Planetology, was able to identify minerals under high temperatures such as those that exist on the surface of Venus. It demonstrated the ability of the remote Raman system to identify atmospheric constituents without landing on the harsh Venusian surface.

In many ways, this genius discovery has proved to be a big boost to the world of science.