The theme for this year's National Science Day is “integrated approach in science and technology for a sustainable future". In this context, the Raman effect is a very good example as to how discoveries in the sciences (physics) can be used for a variety of technological applications, such as in the materials sciences, chemical and biochemical areas, pharmaceutical industry, and medicine.
Raman’s interest in the scattering of light by molecules that ultimately led to the discovery of the Raman effect was triggered during his return journey to India by sea in 1921, after his first foreign trip to England as a Palit Professor at Calcutta University.
Prior to this, his major contributions had been in the field of acoustics and study of musical instruments.
While on board the ship, Raman noticed the dark blue colour of the sea, but was not satisfied by the explanation by Lord Rayleigh that it was due to the reflection of the blue colour of the sky.
Lord Rayleigh had explained the blue colour of the sky as due to scattering of light by molecules in air. These molecules, being much smaller in size compared to the wavelength of light, scatter light by different amounts depending on the wavelength.
The scattered intensity of light is directly proportional to the fourth power of the frequency. Consequently, blue light is scattered more than red. Hence the blue colour of the sky. This kind of scattering is called Rayleigh scattering and is an example of elastic scattering, where the frequency of the scattered light does not change.
Raman conducted many experiments to study the light scattered by sea water using instruments he had with him during his journey and published a couple of papers in the prestigious journal Nature by the time he reached India. He later published a book titled Molecular Diffraction of Light in 1922, reviewing the various aspects of scattering of light by gases, vapours, liquids, crystals, and amorphous solids, as was known until then.
He continued with his experiments on the scattering of light with his students in Calcutta, which eventually led to the discovery of the effect, later named after him.
The Raman effect is an example of inelastic scattering, where the scattered light can have a higher or lower frequency, unlike Rayleigh scattering.
The effect seen by Raman could be explained using the then newly proposed idea of light quanta. This was first used by Albert Einstein to explain the phenomenon of photoelectric effect. Later, this same idea was used by Arthur Compton to understand the Compton effect in 1923.
Compton effect is an inelastic scattering of X-rays by electrons, where the incident X-rays' wavelength increases after its interaction with the electrons. Raman applied these new quantum ideas boldly to understand the newly discovered effect. This idea of inelastic scattering was predicted theoretically by Adolf Smekal in 1923, but wasn’t observed till 1928.
Raman and his colleagues had observed inelastic scattering in a variety of liquids. Around the same time, G S Landsberg and L I Mandelstam had also observed a similar effect in inorganic crystals (“the Raman effect” is still called “combination scattering” in Russia).
We can understand the emission of light of smaller frequency (called Stokes Line) and larger frequency (called anti-Stokes line) in the Raman effect by looking at an energy transition from a lower energy state to a higher virtual energy state, as shown in Figure below.
Virtual states are intermediate energy levels where the electrons from a lower state of an atom or molecule could be excited for a very short time. These virtual states should be below the electronic excited state so that the molecule does not absorb the incident photons completely.
The electronic ground state, from where the electrons are excited by absorption of light, are further divided into sub-levels called the vibrational energy states. These vibrational levels occur in all molecules due to the quantisation of the vibrational energy of the molecule.
When a ground state electron in a particular vibrational level absorbs the incident photon, it gets excited to the short-lived virtual state. When the electron returns back to the ground state, it can result in three possibilities.
First, it comes back to exactly the same level as the original ground state, resulting in what we call Rayleigh scattering (elastic scattering), emitting light of the same frequency υ0.
It can also come back to a higher vibrational ground level, emitting a lower frequency photon (υ0 – υ1). This is called the Stokes line.
And, finally, it can come back to a lower ground state vibrational level, emitting light of higher frequency (υ0 + υ1), called Anti-Stokes line.
The experimental scheme to measure the different frequencies (initially only the Stokes line was identified) due to inelastic Raman scattering effect is shown here.
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