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Class notes Raman Spectroscopy Analytical Instrumentation, ISBN: 9780470518557
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Lecture notes study book Analytical Instrumentation of Gillian McMahon - ISBN: 9780470518557 (.)
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RamanUNIT 4 RAMAN SPECTROSCOPY Spectroscopy
Structure
4.1 Introduction
Objectives
4.2 Theory of Raman Spectroscopy
Quantum or Particle Theory
Classical or Wave Theory
Raman Activity of Vibrations
Rule of Mutual Exclusion
Depolarisation Ratio
4.3 Instrumentation for Raman Spectroscopy
Radiation Sources for Raman Spectroscopy
Sample Handling Devices
Transducers or Detectors
4.4 Enhancement of Raman Spectral Intensities
Resonance Raman Spectroscopy
Coherent Anti-Stokes Raman Spectroscopy
Surface Enhanced Raman Scattering
4.5 Applications of Raman Spectroscopy
4.6 Summary
4.7 Terminal Questions
4.8 Answers
4.1 INTRODUCTION
In the previous unit you learnt about IR spectrometry in terms of its principle,
instrumentation and applications. You would recall that the IR spectrum is a
consequence of transitions amongst the quantised vibrational energy levels of the
molecules on absorbing IR radiation. In this unit you would learn about Raman
spectroscopy which also involves same energy levels but it differs on many other
counts. The nature of radiation used, the mechanism of interaction between the
radiation and matter, the necessary condition for the interaction, the selection rules, the
sensitivity, required instrumentation and the resulting spectra are different in case of
Raman spectroscopy. Further, even the information available from it is different; in
fact it is complimentary to the one available from IR spectroscopy.
As in the previous unit, we shall begin by understanding the theory behind Raman
spectroscopy in terms of the origin of the spectrum and its characteristic features. It
will be followed by an account of the essential components of Raman spectrometers.
Thereafter we shall take up the applications of Raman spectrometry in diverse areas.
In the next block you will learn about spectrometric methods based on molecular
fluorescence and molecular phosphorescence which are important spectroscopic
method of analysis.
Objectives
After studying this unit, you should be able to:
• define Raman effect,
• explain the origin of Rayleigh scattering,
• describe Raman effect in terms of classical wave theory and quantum theory of
radiation,
• define Stokes and anti-Stokes lines and explain their origin,
• compare and contrast Raman spectra with IR spectra,
• state the rule of mutual exclusion and explain its significance,
83
,Molecular Spectroscopic • enlist the essential components of the instrument required for Raman
Methods-I
spectroscopy,
• describe advantages of Raman spectroscopy, and
• enumerate and discuss various applications of Raman spectroscopy.
4.2 THEORY OF RAMAN SPECTROSCOPY
An electromagnetic radiation when passed through a transparent medium interacts
with the particles (e.g., molecules, atoms, or ions) constituting it. If the dimensions of
the particles are equal to or smaller than that of its wavelength then it undergoes
scattering. It has been observed that most of the scattered radiation has exactly the
same wavelength as that of the incident radiation. Such a scattering is referred to as
Rayleigh scattering. However, a very small fraction (to the tune of about 1 in 107) of
the scattered radiation is found to have a wavelength different from that of the incident
radiation. This is called Raman scattering and the existence of Raman scattering is
called Raman effect. Fig 4.1 gives the spectra of the scattered radiation obtained
when a sample of CCl4 was interacted with a laser beam having a wavelength of
488.0 nm.
C.V. Raman ‒ the
discoverer of Raman
Effect in 1928. He was
awarded the 1930 Nobel
prize in Physics. The
significance of his
discovery can be gauged
from the fact that it holds
the record for the shortest
time from a discovery to
awarding of the Noble
prize.
Fig. 4.1: Raman spectra for CCl4 using 488 nm laser source
You may note that the intense peak in the centre of the spectrum has the same
wavelength (or wavenumber) as that of the incident radiation. This is the Rayleigh
Like IR spectrum, the peak and the other signals on either side of the Rayleigh peak are the Raman lines.
Raman spectra are also The lines to the left of Rayleigh peak and having lower value of wavenumber are
given in wavenumber called Stokes lines while the ones to the right and having higher value of wavenumber
units. This is convenient are called anti-Stokes lines. Stokes lines are at lower energy while the anti-Stokes
because the position of lines are at energy greater than the Rayleigh peak.
other scattered radiations
(Raman scattering) can be
The positions of Raman lines are expressed in terms of Raman shift, ∆ν which is
conveniently expressed in
terms of Raman shifts. defined as per the following equation.
∆ν = ( ν s − ν 0 ) cm −1 … (4.1)
Where, ν s and ν 0 are the wavenumbers of the source (or incident) radiation and the
observed scattered lines respectively. It is obvious that the Raman shifts of the Stokes
lines would be positive while for anti-Stokes lines, these would be negative.
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, There are two more features in the spectrum given above. Locate and write them in the Raman
space provided below. Spectroscopy
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
…………………………………………………………………………………………...
We are sure that you could notice that, firstly, the Stokes and anti-Stokes lines are
equidistant from the Rayleigh line and secondly, the Stokes lines are much more
intense as compared to the anti-Stokes lines. Let us understand the mechanism of
origin of the Raman lines, the reasons for the Stokes and anti-Stokes lines being
equidistant from Rayleigh line and their relative intensities.
There are two theories of Raman effect. These are as follows:
• Quantum or particle theory
• Classical or wave theory
Let us discuss these one by one.
4.2.1 Quantum or Particle Theory
You know that according to the quantum theory of radiation, an electromagnetic
radiation is considered to be consisting of a stream of particles called photons. The Elastic collisions: The
photons constituting a radiation of frequency ν would have energy equal to hν where collisions that do not
involve any exchange of
h is the Planck’s constant. The interaction of the radiation with the interacting species
energy.
can be visualised in terms of collisions between them and the photons. If the collisions
are elastic (i.e., they do not involve any exchange of energy) the photons would be
scattered (deflected) with their incident frequency remaining unchanged. This explains
the observance of Rayleigh line in the spectrum. Since most of the collisions are
elastic in nature, most of the scattered photons would have same frequency as the
incident frequency. Therefore the Rayleigh line is observed to be quite intense.
A small fraction of the collisions between the photons and particles of matter is found Inelastic collisions: The
to be inelastic in nature i.e., these involve exchange of energy. When the photons collisions that involve
constituting the radiation undergo inelastic collisions with the absorbing species, they exchange of energy.
either gain or lose energy. These energy exchanges bring about transitions in the
quantised energy levels of the molecules. In such an event of inelastic collisions, the
molecules are either vibrationally (and/or rotationally) excited or they may undergo
vibrational (and/or rotational) relaxation. In both the cases the photons get scattered
with a frequency different from their initial frequency. In former case i.e., when the
molecules undergo excitation, they are of lower frequency. In the later case, where the
molecules undergo relaxation, the scattered photons are of a higher frequency. These
scattered photons give rise, respectively to the Stokes and anti-Stokes Raman lines in
the spectrum.
Let us understand the mechanism of Raman and Rayleigh scattering with the help of
an energy level diagram. Fig. 4.2 gives a schematic energy level diagram of a
molecule. The set of lines at the bottom end represent the electronic ground state and
the corresponding vibrational energy levels. Similarly the set of lines at the top end
represent the first electronic excited state and the corresponding vibrational energy
levels. The region of continuum in the middle represents virtual states whose energies
are not quantised.
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