Light

Raman Scattering

Raman scattering causes the production of scattered photons with a different frequency and this depends on the source as well as on the rotational and vibrational properties of the scattered molecules. In the year 1928, C.V.Raman made a discovery of the inelastic scattering of photons from the molecules such that their excitement takes place to higher levels known as Raman scattering or Raman effect.

Introduction to Raman Scattering

Experts define Raman scattering as the scattering of photons by the molecules whose excitement takes place to the higher energy levels. Experts also call it the Raman Effect. Moreover, inelastic scattering of the photons takes place, which means that an incident particle’s kinetic energy is either lost or increased and comprises of Stokes and anti-Stokes portions.

Inelastic scattering of photons is similar to inelastic collision, which states that total conservation of the microscopic kinetic energy does not take place. In an elastic collision, there will be a transfer of kinetic energy but the scattering will be inelastic.

raman scattering

Raman Scattering Theory

The Raman scattering theory is as follows:

Molecular Vibrations

Raman scattering provides information about vibrations taking place within a molecule. In the case of gases, experts can gather information about rotational energy.

For solids, observation of phonon modes may also take place. Furthermore, there is an application of basics of infrared absorption regarding molecular vibrations to Raman scattering but the rules of selection are different.

Degree of Freedom

Experts define the degree of freedom (DOF) as the number of independent parameters that leads to the determination of the configuration of the physical system. Below is the degree of freedom formula:

DF = n – 1

Where,

DF is the degree of freedom

n shows the number of samples given

In Raman scattering, for any given chemical compound, 3 N is the DOF, where N refers to the number of atoms in the compound. Also, the reason why 3 N is DOF is that each atom’s movement happens in x, y, and z-direction. Also, they possess vibrational, rotational, and translational motion.

Vibrational Energy

It is known that molecular vibrational energy is quantized. Furthermore, its modeling can take place by making use of the quantum harmonic oscillator (QHO) approximation or a Dunham expansion. The vibrational energy levels in accordance to the QHO are

\(E_{n} = h\left ( n + \frac{1}{2} \right )v = h\left ( n + \frac{1}{2} \right )\frac{1}{2\pi }\sqrt{\frac{k}{m}}\)

where n is a quantum number and has the characteristics of quantum numbers.

Raman Spectroscopy

The discovery of Raman spectroscopy took place by C.V.Raman in the year 1928 to study the low-frequency, vibrational, and rotational modes of the molecules. Furthermore, it finds application mainly in chemistry to gather the information that has relation to the fingerprints.

The principle behind Raman spectroscopy is that the passing of monochromatic radiation takes place via the sample such that the reflection, absorption, or scattering of radiation may take place. The frequency of scattered photons is different from the incident photon because of the variation of vibration and rotational property. Consequently, this results in the change of wavelength which is studied in the IR spectra.

Raman shift refers to the difference between the incident photon and the scattered photon. When the energy, whose association is with the scattered photons, is less than the incident photon’s energy, Stokes scattering is said to have taken place. In contrast, anti-Stokes scattering is when the energy of the scattered photons is more than the incident photon.

FAQs For Raman Scattering

Question 1: What are the types of Raman spectroscopy?

Answer 1: The types of Raman spectroscopy are as follows:

  • Resonance Raman Spectroscopy (RRS)
  • Surface-enhanced Raman Spectroscopy (SERS)
  • Micro-Raman Spectroscopy
  • Non-linear Raman Spectroscopic Techniques

Question 2: Differentiate between Rayleigh and Raman Scattering?

Answer 2: When light encounters molecules in the air, elastic scattering is the predominant mode of scattering, which is called Rayleigh scattering. Furthermore, this scattering causes the blue colour of the sky. Moreover, its increase takes place with the frequency’s fourth power and is more effective at short wavelengths.

It is also possible for the incident photons to interact with the molecules in such a way that either the gain or the loss of the energy takes place so that there is shifting of scattered photons in frequency. Most noteworthy, Raman scattering refers to such inelastic scattering.

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