Raman Scattering: Degrees of Freedom, Raman Effect, and Raman Spectroscopy

Raman Scattering, also known as Raman Effect, refers to the phenomenon wherein the inelastic scattering of the photons takes place by the matter wherein the scattered photon’s frequency is different from that of the incident photon. It means that there will be a change in the energy level and the direction of the light as the photon hits the surface of the matter. The phenomenon involves a molecule gaining vibrational energy as incident photons from a visible laser are shifted to lower energy. Raman scattering was discovered by Sir C.V. Raman with the assistance of his student K.S. Krishnan Dr. Raman was awarded Nobel Prize in Physics for discovering the same in the year 1930. In this article, we are going to learn about Raman Effects, Raman scattering, degrees of freedom, and uses of Raman effects.

Key Terms: Raman Scattering, formula of Degree of Freedom, Supercontinuum, wavenumbers, Photon, vibrational energy, molecule

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Raman Scattering

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When light encounters molecules in the air, it gets scattered as soon as it hit the surface of the molecule. The predominant mode of scattering is ‘elastic scattering’ which is also known as Rayleigh Scattering. Here, the energies of the protons are not changed. However, there can be a situation where the incident photon interacts with the molecule in a way that energy is either gained or lost and the photons shift in their frequency. Such inelastic scattering is called the Raman Scattering.

Only a very small fraction of the photons can be scattered inelastically (1 in 10 million approximately). The scattered photons usually have a change in their energy level (usually lower energy), these are the Raman Scattered Photons.

Raman Scattering

Raman Scattering depends on the polarity of the molecule. The incident photon excites the vibrational modes of the molecules, yielding scattered photons with lower energy. Spectral analysis of scattered light will show spectral satellite lines below the Rayleigh scattering peaks at the incident frequency. Such lines are called ‘Stokes lines.’ Although if there is a significant level of excitation of the vibrationally excited states of the scattering molecules, it is possible to see scattering at the frequencies above incident frequency as the vibrational energy is added to the photon incidental energy. These like are generally weaker and called as ‘anti-stokes line’.

Raman Scattering and Energy

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Degree of Freedom

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 Degree of freedom or DoF can be defined as a number of independent parameters that determine the configuration of that physical system. The formula of Degree of Freedom is

DF= n-1

[ Where DF = Degree of Freedom and N = No. of samples taken]

For every given molecule there is a total 3N degree of freedom (N = number of atoms). This number comes from the ability of the molecule to move in all three directions. This 3N degree of freedom is partitioned into three motions namely transitional, rotational and vibrational motions.

Three degrees of freedom correspond to the translational motion of the molecule itself along the three spatial dimensions. Linear molecules have two rotations only as the bond axis does not change the positions of the atoms in the molecule. The remaining degrees of freedom correspond to the vibrational modes of the molecule.

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Raman Spectrum

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The spectrum of the scattered photon is called the Raman spectrum. Here, the wavelength of the scattered photon is converted to wavenumber and then the wavenumbers are plotted on the x-y plane. The wavenumbers are taken along the x-axis along with Raman intensity, plotted on the y-axis. The difference between the wavenumbers and the intensity is measured in Raman Spectrum. A Raman Spectrum shows a number of peaks showing the intensity and wavelength position of the scattered light. The individual peak in the spectrum corresponds to a specific molecular vibration that includes individual and group bonds.

Raman Spectra

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Raman Spectroscopy

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Raman Spectroscopy is a technique for chemical analysis that is non-destructive in nature, used to analyze the chemical structure of the molecule. Needless to say, it employs Raman scattering to study such chemical entities. Sir C.V Raman discovered Raman Spectroscopy to study vibrational, rotational, and low-frequency modes.

Raman Spectroscopy

Principals of Raman Spectroscopy

When the monochromatic light traverses any chemical sample, it may have three fates: firstly it may get reflected, secondly, it may get absorbed, and thirdly it may get scattered. If the scattering is inelastic, the scattered photons will have different energy levels and frequencies than the incident photon. This will result in a change of the wavelength which is known as the Raman shift. This is studied in the IR spectra. Raman spectrum is more like a chemical fingerprint of a molecule that is easily identifiable. The following information can be obtained from a Raman Spectrum:

• Chemical structure and identity
• Phase and polymorphism
• Intrinsic stress/strain
• Contamination and impurities

Principals of Raman Spectroscopy

Types of Raman Spectroscopy

There are various types of spectroscopies where Raman scattering is used. The following are the four types where Raman Effect is used-

• Resonance Raman Spectroscopy (RRS)
• Micro Raman Spectroscopy
• Surface-Enhanced Raman Spectroscopy (SERS)
• Non-Linear Raman Spectroscopic Techniques

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Raman Spectrometer

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Raman Spectrometer is the instrument that employs the principle of Raman Scattering to characterize different molecules. Here a monochromatic light is created with the help of a prism and a weak light is detected with help of the detector. Thus, a spectrum is obtained which is further analyzed to understand the scientific information.

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Application of Raman Effect

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There are several applications of the Raman effect. Some of them are as follows-

• Raman amplification- It is based on Stimulated Raman Scattering or SRS. Here, a lower frequency signal photon induces Raman scattering of a higher frequency pump-photon in an optical medium in a non-linear regime. This method is vividly used in telecommunications.
• Supercontinuum generation- Supercontinuum can be formed using the Raman spectrum. It is widely used in broadband telecommunications and imaging applications.
• Raman scattering is widely used in Nanotechnology to understand the structures of nanowires and other characteristics. It is also used widely in medicine and biochemistry to study the chemistry of the samples.
• It has applications in remote sensing and astronomical planetary exploration.

Also Read:

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Things to Remember

• Raman Effect is the phenomenon of inelastic scattering of the photons by the matter where the frequency of the scattered photon is different from that of the incident photon.
• For every given molecule there can be a total 3N degree of freedom (N= number of atoms). This number comes from the ability of the molecule to move in all three directions. 
• This 3N degree of freedom is partitioned into three motions namely transitional, rotational and vibrational motions.
• Raman Spectrum shows a number of peaks showing the intensity and wavelength position of the scattered light. 
• The individual peak corresponds to a specific molecular vibration including individual bonds and the group of bonds.
• Some of the various information that can be obtained by Raman Spectroscopy are- Chemical structure and identity, phase and polymorphism, intrinsic stress/strain, Contamination, and impurities of a sample.
• Raman spectrum is used in different fields including telecommunications, nanotechnology, microscopy, and biology.

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Previous Year Questions  

1. When hydrogen atom is in its first excited level, its radius, is….
2. When an electron does transition from n=4 to n=2 , then emitted line spectrum will be….​..
3. α -particle consists of….
4. An electron of a stationary hydrogen atom passes from the fifth energy level….
5. Complete the equation for the following fission process…. [NEET 1998]
6. using non-relativistic approach, the speed of electron in this orbit will be…. [NEET 2015]
7. When the glancing angle of incidence of light on a material is...[COMEDK UGET 2004]
8. Two waves having intensity ratio 25 : 4 produce interference. The ratio of maximum to minima intensity is...[COMEDK UGET 2004]
9. In Young's double slit experiment,1^st dark fringe occurs directly...[COMEDK UGET 2009]
10. In the diffraction pattern due to a single slit linear width of the central max...[COMEDK UGET 2007]
11. In Newton ring experiment, monochromatic light is replaced by white light...[COMEDK UGET 2008]
12. In diffraction through a single slit experiment, slit width is halved...[COMEDK UGET 2004]
13. Which of the following is false for interference of light?...[JKCET 2012]
14. Which of the following is true for the minimum angular separation of two stars...[JKCET 2012]
15. Values for Brewster's angle can be...[JKCET 2015]
16. Unpolarized light falls on two polarizing sheets placed one on top of other….[JKCET 2013]
17. In Young's double slit experiment using monochromatic light of wavelength   \(\lambda\) ….[AMUEEE 2018]
18. In a single-slit diffraction experiment, the width of the slit is reduced by...[JKCET 2012]
19. the wavelength of light illuminating the slits is….[JKCET 2013]
20. Colours in thin films are due to….[JKCET 2008]