To read the article in Odia click here.
The Raman Effect / Raman Scattering
The Raman Effect, also known as the Raman Scattering, is an important discovery made by C.V. Raman and his student physicist K.S. Krishnan. Raman was awarded the Nobel Prize for Physics due to this discovery. The Raman Effect has found vital applications in both Physical sciences and Chemical sciences.
HOW DID C.V. RAMAN DISCOVER THE RAMAN EFFECT?
In 1921, while returning from London to Bombay aboard a ship, Raman was fascinated by the deep blue color of the Mediterranean Sea. He thoughtfully discarded Rayleigh’s explanation that the color of the sea was just a reflection of the color of the sky. Sometime later, he was able to prove that the color of the sea was the result of scattering of sunlight by water molecules. This led to his increased interest in scattering of light by molecules. He then conducted a series of experiments and concluded that there occurred a frequency-shift scattering of the light by molecules. This ultimately led to his discovery of the Raman effect where the frequency-shift is a characteristic of the molecules in the sample and it is independent of the frequency of the incident light. Therefore, it is different from fluorescence. Raman and Krishnan recorded the spectra of the Raman effect experiment using mercury lamp and photographic plates. Most of Raman’s work was concerned with vibrational transitions, which give larger shifts observable for gases, liquids, and solids. Gases have low molecular concentration at ordinary pressures and therefore produce very faint Raman effects; thus liquids and solids were more frequently studied.
DEFINITION :
The change in the wavelength of light that occurs when a light beam is deflected by molecules of a sample (liquid or gas or solid) is known as the Raman Effect.
When a sample of a chemical compound (preferably dust-free and transparent) is exposed to a monochromatic light of the visible region, the sample absorbs light and the major portion of this light gets transmitted through the sample unchanged in direction, frequency, wavelength and intensity. However, a small fraction of the light is scattered by the sample indirections other than the incident direction. The incident light has a particular frequency. If the scattered light has frequency same as that of the incident light (unchanged wavelength), then that scattering is called Rayleigh scattering. But about 1% of the total scattering occurs at frequencies different than the incident frequency (wavelengths different from that of the incident light). This is known as Raman Scattering or the Raman Effect.
MECHANISM :
When an incident photon of the monochromatic light interacts with an electron of a molecule in the sample, the electron absorbs energy from the incident photon and thus rises to a higher state of energy (a higher vibrational level). The energy it gains is the energy of the photon and is directly proportional to the incident frequency (E=hv, v is the incident frequency). The electron then falls back to the lower energy level by losing energy. If the energy lost is equal to the energy of the incident photon, then the electron falls back to its initial level and in this process emits another photon. This emitted photon has the same frequency and the same wavelength as that of the incident photon. This is the scattering of the light in different directions without any change in frequency and wavelength. Hence, it is Rayleigh scattering. But sometimes the electron, when losing energy from the higher energy state, falls back to a different energy level. Therefore, in this case, the energy lost by the electron is different than the energy absorbed from the incident photon. As a result, the photon emitted has energy different from that of the incident photon. This gives rise to scattering of light with changed frequency and wavelength. This is Raman scattering.
Depending on the final energy level (the final vibrational level) of the electron, Raman scattering is separated into Stokes line (when scattering frequency is less than the incident frequency, in other words, electron absorbs energy from photon) and Anti-Stokes line (when scattering frequency is more than the incident frequency, in other words, electron releases energy to the photon).
 |
| Credits: insightsonindia |
For a liquid compound, intensity of the affected light is 1/1,00,000 of that of the incident light. This shows that the Raman effect is very feeble.
RAMAN SPECTROSCOPY :
Raman effect gives rise to Raman Spectroscopy. The Raman spectra, which is recorded as a result of the Raman scattering, gives the Molecular Fingerprint of the sample and it is different for different molecules. By studying the spectra, one can identify rotational levels and hence in turn can identify a particular molecule in the sample. This helps in performing the Qualitative Analysis (identifying the constituent molecules in the sample). Similarly, the intensity of a particular Raman line helps to determine the concentration of a molecule in the sample. In this manner, Quantitative Analysis is done. Hence, the pattern of the Raman lines is characteristic of the particular molecular species, and its intensity is proportional to the number of scattering molecules in the path of the light. Thus, Raman spectroscopy is used in qualitative and quantitative analysis of chemical compounds and of solids (determining the crystallographic orientation). Highly complex materials like biological organisms and human tissues can also be analysed using Raman Spectroscopy.
OTHER APPLICATIONS OF RAMAN EFFECT :
Raman LIDAR is used in atmospheric physics to measure the atmospheric extinction coefficient and the water vapor vertical distribution. Raman amplification is used in optical amplifiers. Hence, C.V. Raman’s work was pathbreaking and highly significant for development of science.
Biography of C V Raman
 |
| Creator: 2001 SNOWBOUND, Credit: OSA Photo Archive Copyright: OSA Photo Archive |
He was a studious child from the beginning and completed his secondary and higher secondary education from St Aloysius' Anglo-Indian High School at the ages of 11 and 13, respectively. He then topped his BA degree at the age of 16 from the Presidency College of University of Madras with gold medals in Physics and English (year 1904). At the age of 18 only, year 1906, while still being just a graduate student, he published his 1st scientific paper on “Unsymmetrical diffraction bands due to a rectangular aperture” in a British journal Philosophical Magazine. This became the 1st ever research paper published from the Presidency College. In the year 1907, at the age of 19, he obtained his MA degree with highest distinction from the same university. In the same year, he published his 2nd scientific paper in the same journal. It was due to this paper, that he started to communicate with Lord Rayleigh, a prominent scientist of that time.
In the year 1907, Raman cleared the entrance examination of Indian Finance Service with the 1st position and was appointed as the Assistant Accountant General in Calcutta. It was during this time that he came in contact with people of University of Calcutta and IACS (Indian Association for the Cultivation of Science, the 1st research institute of India). Due to this, Raman started doing his research in IACS, even after being a full-time civil servant. In the same year (1907), he published another research article “Newton’s rings in polarized light” in Nature from the IACS. Due to this, he became the major contributor for the research journal published by IACS. He received research prizes in 1912 and 1913 while he was a full-time civil servant.
In the year 1915, the University of Calcutta started assigning research scholars under Raman at IACS. In the year 1917, Raman sacrificed his civil service job to become a full-fledged researcher and professor in the University of Calcutta. In 1919, Raman was appointed as an Honorary Professor and the Honorary Secretary at IACS. He used IACS as the research arm of the University of Calcutta.
In the year 1921, Raman went abroad to Oxford to deliver a lecture due to which he was then widely known and interacted with great scientists like J.J. Thomson and Rutherford. Due to his good research progresses, he was elected as a Fellow of the Royal Society in the year 1924. The same year, he toured the United States and spent 4 months at the prestigious California Institute of Technology through the invitation of Nobel Laureate Robert Millikan. He became popular in India and abroad among the scientific population due to his booming voice, lively demonstrations, superb diction and rich humor other than his wonderful research works.
In 1926, he started Indian Journal of Physics as its 1st editor. In the 2nd volume of this journal, he published his article “A New Radiation” which reported the discovery of the RAMAN EFFECT. This work of his was also published in Nature. This pathbreaking research work by him yielded him the Nobel Prize for Physics in the year 1930 thus making him the 1st Asian to win the Nobel in Sciences.
In 1933, Raman left Calcutta and went to Bangalore to become the Director of Indian Institute of Science (IISc), which is the premier research institute of India currently. In 1947, he was appointed as the 1st National Professor of Independent India. In 1948, he established Raman Research Institute in Bangalore after retiring from IISc. In the year 1954, he was awarded the Bharat Ratna. He served as Director of Raman Research Institute and remained active there both in research and teaching until his death in 1970. He died on the 21st of November 1970 at the age of 82 in Bangalore.
February 28, 1928 was the day on which Raman, along with his student physicist K.S Krishnan, discovered the Raman Effect which 2 years later became the reason for Raman’s Nobel. Therefore, 28 February is celebrated as National Science Day every year in India in the honour of C.V. Raman’s discovery of the Raman Effect.
The American Chemical Society (ACS) and the Indian Association for the Cultivation of Science (IACS) dedicated The Raman Effect an International Historic Chemical Landmark on December 15, 1998 at the Indian Association for the Cultivation of Science in Jadavpur, Calcutta, India because of its vital use in in the field of Chemical sciences any not only in Physical sciences.
- https://en.wikipedia.org/wiki/Raman_scattering
- https://www.britannica.com/science/Raman-effect
- https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/ramaneffect.html
- https://www.osa-opn.org/home/articles/volume_20/issue_3/features/c_v_raman_and_the_raman_effect/
- https://en.wikipedia.org/wiki/C._V._Raman
Picture Sources :
- https://www.optica.org/en-us/history/biographies/bios/c-v--raman/
- https://www.insightsonindia.com/2020/07/07/what-is-raman-spectroscopy/
