Study Notes On Raman Spectrum (Chemical Science) - Download PDF!

By Astha Singh|Updated : November 26th, 2022

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Candidates preparing for their CSIR NET exam can really make their preparation journey easier with the help of some reliable study notes that cover the topics in the most simple way. We at BYJU'S Exam Prep have come up with the idea of providing short notes on Raman Spectrum, which comes under the Physical Chemistry section of the Chemical Science syllabus.

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Study Notes On Raman Spectrum

The Raman effect takes place when a photon is incident on a molecule and undergoes an interaction with the electric dipole of the molecule. In quantum mechanics, scattering refers to excitation to a virtual state lower in energy than a real electronic transition with nearly coincident de-excitation and a change in vibrational energy. The energy level diagram which depicts the Stokes and Anti-stokes Raman scattering is given below:

The source, sample, and detector are situated at 90° of each other.

Raman Activity: For a molecule to be Raman active, there must occur some change in a component of molecular polarizability. This entity is an anisotropic property and can be expressed as:

Here,

α = Polarizability

E = Electric field of radiation

μ = induced dipole moment.

All the molecules with the centre of symmetry show mutual exclusion between I.R. and Raman activity.

If a molecule has a centre of symmetry, then its IR active vibrations will be Raman inactive, and Raman active vibration will be IR inactive.

RAMAN EFFECT AND RAMAN SCATTERING

When electromagnetic radiation with frequency and energy strikes the molecule of the sample, then two possibilities are:

(1). In the case of a perfectly elastic collision, the photon will deflect as such without any change in energy. This phenomenon is known as Rayleigh scattering.

(2). In the case of inelastic collision, molecules can either gain or lose energy related to vibrational and rotational energy levels. This phenomenon is known as Raman scattering. If there is a gain of energy takes place, then the energy of the scattered photon will be as:

Here, ΔE is the Raman shift.

For pure rotational Raman spectra, the value of Raman shift ΔE will be equal to the separation in rotational energy levels.

For vibrational Raman spectra, the value of Raman shift will be equal to the separation in vibrational energy levels.

The radiation that scatters with a frequency lower than that of incident frequency is known as stokes radiation.

Radiation scattered with a frequency higher than that of incident frequency is known as Anti stokes radiation.

In Raman scattering, if photons get scattered by a ground state molecule, then loss in energy takes place which is equal to rotational or vibrational energy level separation and scattered photons have energy less than incident photons. This is known as Stokes radiation. If a scattered photon has higher energy than that of the incident photon, then radiation is anti-stoke.

Since the population of molecules in the ground state of rotational and vibrational energy levels are always higher than that of the excited state, as a result, the intensity of stoke lines will always be higher than anti-stokes lines.

The position of stoke and anti-stokes lines are a mirror image of each other from the position of incident radiation.

Pure Rotational Raman Spectra (For linear molecule):