In 1928, the famous Indian scientist Sir Chandrashekhar Venkat Raman, or Sir C.V. Raman, passing monochromatic visible light rays through various liquids (e.g., benzene, toluene, etc.), observed that the incident rays were immediately scattered, in addition to the main wavelength radiation, with relatively shorter and longer wavelength radiation. This phenomenon is called Raman effect. In addition to liquids, solid and gaseous media also exhibit this action.
Rayleigh's Scattering Law
The intensity of scattered light (I) is inversely proportional to the fourth power of the wavelength (λ) of the light.
According to this formula, smaller particles scatter shorter wavelength light (blue, violet, etc.) more than longer wavelength light (red, etc.).
Rayleigh scattering law does not apply when the particle size is much larger than the wavelength of the incident light.
According to the Rayleigh scattering law, the intensity of scattered light (I) is proportional to the sixth power of the diameter (d) of the dust particles floating in the atmosphere.
Characteristics of Raman Effect
- In addition to the main spectral line produced by the Raman effect, several faint spectra of shorter and longer wavelengths are seen. Lines of longer wavelength (or lower frequency) are called ‘Stokes lines’ and lines of shorter wavelength (or higher frequency) are called ‘anti-Stokes lines’. In the main spectral line part of the Raman spectrum, the frequency of the incident radiation and the scattered radiation are equal. This scattering of light in the Raman spectrum without any change in frequency is called Rayleigh scattering, and this spectral line is called Rayleigh line. Anti-Stokes lines, Rayleigh lines, and Stokes lines are collectively called Raman lines.
- Strokes lines and anti-Stokes lines are spaced at equal frequency intervals on either side of the main line. The frequency of these lines depends on the frequency of the main line.
- The frequency difference (Δν) of the Strokes line and anti-Stokes lines from the baseline does not depend on the frequency of the baseline. Δν depends on the nature of the substance under test. The frequency gap Δν is called the Raman shift. If the frequency of the original line is ν0, then the frequency of the Strokes line and the anti-Stokes line are, respectively,
- The intensity of Raman lines is generally lower than that of the mainline. Stokes lines are more intense than anti–Stokes lines.
- Raman lines are generally convergent.
Explanation of Raman effect by Quantum theory
According to quantum theory, any radiation is considered a stream of photons with energy hν. When such a light photon hits a solid, liquid, or gaseous particle, three types of collisions can occur between the photon and the particle.
- A photon can be scattered from its path without increasing or decreasing its energy or frequency. The result will be an inverted spectrum of the same frequency as the incident light.
- The photon imparts part of its energy to the molecule at ground level, causing the molecule to move to a higher energy level, and the photon’s energy decreases.
- If the molecule is already at a higher energy level, the photon gains some energy from the molecule, and the molecule loses energy and transmits to the ground level.
In the first case, elastic collisions occur between protons and molecules. So, the frequency of the scattered photon is equal to its initial frequency. So, this explains the existence of Rayleigh lines in the Raman spectrum.
In the second and third cases, inelastic collisions occur between protons and molecules. This results in two situations. The frequency of the photon can decrease (increase in the Stokes line) or increase (increase in the anti-Stokes line).
Let the intrinsic energy of the molecule in its initial state = Ep, and the intrinsic energy of the molecule after collision = Eq. Mass of molecule = m, velocity of molecule before collision = v, velocity of molecule after collision = v’, Energy of incident photon = hν, Energy of scattered photon = hν’. According to the conservation of energy formula,
Since the collision does not significantly change the temperature of the particle’s surroundings, it can be assumed that the kinetic energy of the particle remains unchanged as a result of the collision. Hence, from equation no. (1),
Now, 1. If Ep = Eq, then ν’ = ν; that is, the frequency of the scattered radiation is equal to the primary frequency. This indicates the Rayleigh line. 2. If Ep < Eq, then ν’ < ν; that is, the frequency of the scattered radiation decreases and Stokes lines are formed. 3. If Ep > Eq, then ν’ > ν; that is, the frequency of the scattered radiation increases and anti-Stokes lines are formed.
Application of Raman effect
- This function is used to determine the molecular structure of various compounds and molecules.
- Information about the atomic arrangement of a particular molecule is known with the help of the Raman effect.
- Information about the spin of the nucleus is also obtained with this function.