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Depending on the source and the rotational and vibrational properties of the scattered molecules, Raman scattering produces scattered photons with a different frequency. Raman scattering is the basis for the principle of Raman spectroscopy. It is useful for the study of materials by physicists and chemists. In former times, mercury lamps and photographic plates were used for the recording of the spectra. In modern times, lasers are useful. C. V. Raman, along with his student K. S. Krishnan discovered Raman’s scattering (Raman shift). For this discovery, Sir C. V. Raman was granted the Nobel prize in the field of Physics in 1930.
Raman scattering is the scattering of photons by excited molecules at higher energy levels. It is otherwise called the Raman effect. The kinetic energy of the incident particle is increased or decreased and is constituted by Stokes and anti-Stokes portions, where photons are inelastically scattered.
The concept of inelastic scattering of photons is identical to the concept of inelastic collision, where total microscopic energy is not conserved. But in elastic collision, transfer of kinetic energy takes place.
Analogous to Rayleigh scattering, Raman scattering too is dependent on the polarizability of the molecule. In comparison with the intensity of the excitation source, the intensity of Rayleigh scattering is around 10-3 to 10 -4. After the scattering, the energy of the photon and the state of the molecule is unchanged. But in Raman scattering, the frequency of photons present in the monochromatic light changes during interaction with the vibrational modes or states of a molecule.
The laser acts as an intense monochromatic light in Raman scattering. It can give rise to scattered light containing one or more sidebands, which are made offset by vibrational or rotational energy differences. The produced sidebands are inclusive of frequencies, consisting of information about the scattering medium and so, they are used in remote sensing.
When air is encountered by light molecules, elastic scattering is the predominant mode of scattering, which is also known as Rayleigh scattering. This aspect is the reason behind the blue color of the sky. It rises with the fourth power of the frequency and is extra effective at shorter wavelengths. Another possibility is the energy of incident photons is gained or lost after interaction with the molecules and the scattered photons are shifted in frequency. This inelastic scattering is Raman scattering.
Raman scattering, like Rayleigh scattering, is dependent on the polarizability of molecules. The incident photon is able to excite vibrational modes of the molecules. This yields scattered photons, diminished in energy by the amount of the vibrational transition energies. There is an exposure of satellite lines (Raman lines) below the Rayleigh scattering peak at the incident frequency under the spectral analysis of the scattered light. The lines are called Stokes lines.
Whenever there is considerable excitation of vibrationally excited states of the scattering molecules, there is a probability for observation of the scattering at frequencies higher than the incident frequency. It happens since the vibrational energy is supplemented with the incident photon energy. These lines are usually weaker and are called anti-Stokes lines.
The Raman effect produces no pure rotational spectrum when the molecules exhibit no net dipole moment. This yields information regarding the moment of inertia and the structure of the molecule.
The scattered light consists of one or more sidebands, which are offset by vibrational and rotational energy differences. As the sidebands consist of information about the scattering medium, this is used for identification and remote sensing.
The DOF is specified as the number of independent parameters, which determine the configuration of the physical system.
The formula for DOF is:
DF = n - 1 where
n is the number of given samples
DF is the degree of freedom
3N is the DOF for any chemical compound in the Raman scattering, in which number of atoms in the compound.
3N is the DOF as the every atom moves in z-direction, y-direction and x-direction because they have vibrational rotational and translational motions
Raman spectroscopy was discovered by C.V.Raman in 1928 for studying the rotational, vibrational, and low-frequency of molecules. Its major application in chemistry is for getting the information associated with footprints.
Raman spectroscopy works under the principle that when monochromatic radiation is passed through the sample, the radiation may get scattered, reflected, or absorbed. The scattered photons possess a different frequency from the incident photon because the rotational and vibrational properties vary. This leads to the change in wavelength that is analyzed in IR spectra.
There are so many variants as of now in Raman spectroscopy. Following are some of the important variants of Raman spectroscopy:
Raman spectrometer is one of the equipment, which contains one or more single-colored light sources and lenses and filters for focussing the light and differentiation of reflected and scattered light respectively. A prism is utilized for light splitting into its components and a detector is used to detect the weak light. Then, the spectrum is acquired on the monitor for the analysis of the information.
The wavelength of the scattered photon is transformed into wavenumber for analysing the Raman effect. On the x-y plane, wavenumbers are plotted.The wavenumber is plotted on the x-axis and Raman intensity is put on the y-axis. The plot showing the wavenumbers and intensity is known as Raman spectrum.
Though Raman effect has excess applications, some significant applications have been provided below:
Conclusion
A concise introduction to Raman spectroscopy and Raman scattering have been given. Then, the concept of Raman scattering has been explained. The degrees of freedom have been briefed. Further, the principle of Raman spectroscopy has been described. Matters like types of Raman spectroscopy. Raman spectrometer and Raman spectra have been notified. Finally, applications of the Raman effect have been itemized.
