Understanding
The Raman Effect
Scattering of light Explained
The Raman Effect
The Raman effect, also known as the Raman scattering, is a light scattering that provides remarkable details of the substance that scatters the light. The spectrum that we see as a result of the Raman scattering is a unique fingerprint of that substance.
Overview
In 1928, Indian physicist Sir C.V. Raman (Chandrashekhar Venkataraman) made a profound discovery of a phenomenon which is now known as the Raman effect or the Raman scattering. Soon after his discovery, scientists recognized the immense significance of the discovery because of its tremendous practical applicability as an analytical tool.
For this great discovery, C.V. Raman was awarded the 1930 Noble prize in physics.
It is interesting to note that Sir C.V. Raman was the uncle of the famous astrophysicist Chandrasekhar who was also a Nobel laureate for his discovery of Chandra Limit.
Before exploring the Raman effect, let us first see what led to this discovery.
Background
In 1921, while on the 15-days return-voyage from London to Bombay by ship, the awesome, deep blue Mediterranean Sea made Raman wonder why the sea was looking so blue. A few years earlier, Lord Rayleigh of Britain, after discovering why the sky was blue, reasoned that the blue color of the sea could be just a reflection of the sky. However, Raman’s probing mind could not accept that reasoning. He thought that if scattering of light by air-molecules was the cause for the blue sky, the same scattering phenomenon could be playing a role for the deep blue color of seas.
Soon after reaching India, Raman and his team embarked on the research of scattering of light in various liquids and solids. This research work, at the Indian Association for the Cultivation of Science (IACS) in Kolkata (Calcutta), eventually led to the discovery of the Raman scattering of light.
In order to understand Raman scattering, we need to quickly review the following:
- What is scattering of light?
- What are the types of scattering?
Scattering of Light
Scattering of light occurs when some of the light particles (photons), while passing through a medium, get absorbed by the particles hanging in the medium or by the molecules of the medium, and then get re-emitted in a different direction from their original direction. This phenomenon is known as the scattering of light.
During the light scattering process, the absorption and the re-emission of a photon could happen in two different ways: as elastic scattering or as inelastic scattering.
Elastic Vs Inelastic
In elastic scattering, a hanging particle of the fluid (or a molecule of the medium) first absorbs a photon and then re-emits a photon without making any change to the photons energy. There is no loss of energy in this process. Therefore, there is no change in the wavelengths of the incident ray and the re-emitted ray.
This type of elastic scattering is also known as Rayleigh scattering which is responsible for the blue sky.
On the other hand, in inelastic scattering, a particle of the fluid (or a molecule of the medium) absorbs a photon and then re-emits a photon with a change in its energy. The is known as the Raman scattering. The change in energy can be either positive or negative. If it is negative, it is Stoke Raman scattering and if it is positive, it is anti-Stoke Raman scattering.
Raman Scattering
The picture above illustrates how the light scattering process works when a photon collides with a molecule of a medium. It also shows the differences between the Raman scattering (inelastic scattering) and the Rayleigh scattering (elastic scattering).
Notice that in Rayleigh scattering the wavelength of the incident light (in green) and the wavelength of the scattered light (in green) remain the same. Whereas in the Raman scattering the wavelengths of the scattered light get changed. In Stokes Raman scattering, the wavelength increases (in red) and in anti-Stokes Raman scattering the wavelength decreases (in blue).
Though the intensity of the light from the Raman scattering is substantially less, the Raman scattered light carries lots of information about the scattering substance. Therefore, the Raman scattering has immense applications.
Applications
Though Rayleigh scattering produces more intense light, it does not carry any information about the scattering molecules. However, Raman scattering, though the intensity of the scattered light is much lower, the scattered light carries lots of information about the scattering molecules. The spectrum produced by the Raman effect is a kind of fingerprint of a scattering molecule. That explains why the Raman scattering is used as a powerful tool for analyzing the composition of liquids, gases, and solids.
Sir C.V. Raman in his address in 1930 while accepting the Noble prize said, “… the character of the scattered radiations enable us to obtain an insight into the ultimate structure of the scattering substance.”
Yes, the scattered light through the Raman effect provides much insight into the structure of the scattering substance.
Conclusion
Raman scattering is a weak process – only one in one million photons goes through the Raman scattering. However, the spectrum produced from the Raman scattering provides much insight into the chemical and structural details of the substance that scatters light. That explains why the Raman scattering is being used as a great analytical tool.
Currently, Raman scattering helps from the non-destructive identification of minerals to the early detection of diseases.
References
References:
- Britannica, The Editors of Encyclopaedia. “Raman effect”. Encyclopedia Britannica, 14 Jun. 2024, https://www.britannica.com/science/Raman-effect. Accessed 17 July 2024.
- the-raman-effect-commemorative-booklet.pdf from the Indian Association for the cultivation of science, Jadavpur, Kolkata (Calcutta), India
- https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Instrumental_Analysis_(LibreTexts)/18%3A_Raman_Spectroscopy/18.01%3A_Theory_of_Raman_Spectroscopy
- https://www.edinst.com/blog/what-is-raman-spectroscopy/