Download Raman Spectroscopy

Spectroscopy
By
Dr. Depinder Kaur
Associate Prof.(Chemistry)
P.G.G.C.-11 Chandigarh
SPECTROSCOPY
• Spectroscopy is that branch of
science which deals with the
study of interaction of
electromagnetic radiations with
matter
ELECTROMAGNETIC
SPECTRUM
• When the different types of
electromagnetic radiations are
arranged in order of their increasing
wavelengths or decreasing
frequencies, the complete
arrangement is called
electromagnetic spectrum.
CHARACTERISTICS
• DUAL CHARACTER
• ELECTRIC & MAGNETIC FIELD
• TRAVEL WITH VELOCITY OF
LIGHT
• c =νλ
• E=hv
Region of the
Main interactions with matter
spectrum
Collective oscillation of charge carriers in
Radio
bulk material (plasma oscillation). An
example would be the oscillation of the
electrons in an antenna.
Microwave through Plasma oscillation, molecular rotation
far infrared
Molecular vibration, plasma oscillation (in
Near infrared
metals only)
Molecular electron excitation (including
Visible
pigment molecules found in the human
retina), plasma oscillations (in metals only)
Excitation of molecular and atomic valence
Ultraviolet
electrons, including ejection of the
electrons (photoelectric effect)
X-rays
Excitation and ejection of core atomic
electrons
Energetic ejection of core electrons in
Gamma rays
heavy elements, excitation of atomic
nuclei, including dissociation of nuclei
Creation of particle-antiparticle pairs. At
High energy
very high energies a single photon can
gamma rays
create a shower of high energy particles
and antiparticles upon interaction with matter.
MOLECULAR SPECTROSCOPY
• INTRODUCTION
• DIFFERENCE FROM ATOMIC
SPECTROSCOPY
ABSORPTION AND EMISSION
SPECTROSCOPY
• A molecule has a number of energy levels
which are quantized. The transitions take
place only between these energy levels
according to certain rules, called selection
rules.
• The transition may take place from lower
energy level to higher energy level by
absorbing energy. It is then called
absorption spectroscopy and the result
obtained as a result of a number of such
• The transition may take place from higher
energy level to a lower energy level
thereby emitting the excess energy as a
photon. It is then called emission
spectroscopy and the result obtained as
a result of a number of such transitions is
called emission spectrum.
•
E2 – E1 = hv = hc/ λ
TYPES OF MOLECULAR
SPECTRA
• A molecule has different types of
quantized energy level i.e.
translational, rotational, vibrational
and electronic. The molecular
spectra arise due to transitions taking
place between these energy levels.
• E t <<E r <<E v <<E e*
•
In fact, the difference between the
successive translational energy levels
is so small (~ 10-60 J mol-1) that it
cannot be observed experimentally.
For this reason, for practical
purposes, translational energy is
considered as continuous and we do
not observe any translational
spectrum.
Pure rotational
(Microwave) spectra.
• If the energy absorbed by the molecule is so
low that it can cause transition only from one
rotational level to another within the same
vibrational level, the result obtained is called
the rotational spectrum. These spectra are,
therefore, observed in the far-infra-red
region or in the microwave region whose
energies are exceedingly small (v = 1-100
cm-1). The spectra obtained is, therefore,
also called microwave spectra.
Vibrational rotational
spectra
• If the exciting energy is sufficiently large so
that it can cause a transition from one vibration
level to another within the same electronic
level, then as the energies required for the
transitions between the rotational levels are still
less, both types of transitions will take place.
The result is, therefore, a vibration-rotational
spectrum. Since such energies are available in
the near infra-red region, these spectra are
observed in this region (v = 500-4000 cm-1)
and are also called infra-red spectra.
Electronic Band Spectra
• If the exciting energy is still higher such that it
can result in a transition from one electronic
level to another, then this will also be
accompanied by vibrational level changes and
each of these is further accompanied by
rotational level changes. For each vibrational
change, a set of closely spaced lines is
observed due to rotational level changes. Such
a group of closely spaced rotational lines is
called a band.
• Thus whereas atoms give line
spectra, molecules give band spectra.
As such high excitation energies are
available in the visible and ultraviolet
regions, these spectra are observed
in the visible region (12,500-25,000
cm-1) and ultraviolet region
(25,000-70,000 cm-1).
Nuclear Magnetic Resonance
(NMR) spectra.
• This type of spectrum arises from the
transitions between the nuclear spin
energy levels of the molecule (involving
reversal of nuclear spin) when an external
magnetic field is applied on it. The
energies involved in these transitions are
very high which lie in the radio frequency
regions (5-100 MHz).
Electron Spin Resonance
(ESR) spectra.
• This type of spectrum arises from the
transitions between the electron spin
energy levels of the molecule (involving
reversal of electron spin) when an external
magnetic field is applied on it. These
involve frequencies corresponding to
microwave region (2000-9600 MHz).
Raman spectra.
• This is based on scattering of
radiation and not on the absorption
of radiation by the sample.
RAMAN SPECTRA
GENERAL INTRODUCTION
• It is a type of spectroscopy which deals not with
the absorption of electromagnetic radiation but
deals with the scattering of light by the
molecules. When a substance which may be
gaseous, liquid or even solid is irradiated with
monochromatic light of a definite frequency v, a
small fraction of the light is scattered. Rayleigh
found that if the scattered light is observed at
right angles to the direction of the incident light,
the scattered light is found to have the same
frequency as that of the incident light. This type
of scattering is called Rayleigh scattering.
• When a substance is irradiated with
monochromatic light of a definite
frequency v, the light scattered at right
angles to the incident light contained lines
not only of the incident frequency but also
of lower frequency and sometimes of
higher frequency as well. The lines with
lower frequency are called Stokes' lines
whereas lines with higher frequency are
called anti-Stokes' lines.
• Raman further observed that the
difference between the frequency of the
incident light and that of a particular
scattered line was constant depending
only upon the nature of the substance
being irradiated and was completely
independent of the frequency of the
incident light. If vi is the frequency of the
incident light and vs' that of particular
scattered line, the difference v = vi - vs is
called Raman frequency or Raman
shift.
EXPLANATION FOR
OBSERVING RAYLEIGH LINE
AND RAMAN LINES
• Elastic and inelastic collisions between the
radiations and interacting molecules results in
the formation of Rayleigh and Raman lines.
• In terms of excitation of electrons
•
vabs = ve Rayleigh line
•
vabs > ve Stokes line
•
vabs < ve Anti Stokes line
•
POLARIZABILITY OF MOLECULES
AND RAMAN SPECTRA
• The Raman effect arises on account of the
polarization (distortion of the electron cloud) of the
scattering molecules that is caused by the electric
vector of the electromagnetic radiation. The
induced dipole moment depends upon the strength
of the electric field E and the nature of the
molecules represented by α
•
μ= αE
•
In case of atoms or spherically
symmetrical molecules (spherical rotors)
such as CH4' SF6 etc same polarizability is
induced whatever be the direction of the
applied electric field. They are said to be
isotropically polarizable. Such molecules
are said to be isotropic molecules.
• In case of all diatomic molecules
(homonuclear or heteronuclear) or nonspherical molecules (non-spherical rotors),
the polarizability depends upon the
direction of the electric field.
• Such molecules are, therefore, said to
anisotropically polarizable.
Types of molecules showing
Rotational Raman Spectra.
• A molecule scatters light because it is
polarizable. Hence the gross selection rule
for a molecule to give a rotational Raman
spectrum is that the polarizability of the
molecule must be anisotropic i.e. the
polarizability of the molecule must depend
upon the orientation of the molecule with
respect to the direction of the electric
field.
• Hence all diatomic molecules, linear
molecules and non-spherical molecules
give Raman spectra i.e. they are
rotationally Raman active. On the other
hand, spherically symmetric molecules
such as CH4' SF6 etc. do not give
rotational Raman spectrum i.e. they are
rotationally Raman inactive. (These
molecules are also rotationally microwave
inactive).
PURE ROTATIONAL RAMAN
SPECTRA OF DIATOMIC
MOLECULES
• The selection rules for pure rotational Raman
spectra of diatomic molecules are
ΔJ = 0 , + 2
•
•
• The selection rule
•
ΔJ
= 0 corresponds to
Rayleigh scattering whereas selection rule
ΔJ = + 2 gives rise to Raman lines as
explained .
INTENSITIES OF LINES OF THE
PURE ROTATIONAL RAMAN
SPECTRA
• As explained earlier, the intensities of lines
depend upon the population of initial level
from where the molecules are excited or
de-excited to the final level. Since the
population of rotational energy levels is as
shown in Fig. therefore the intensities of
the Stokes' and anti-Stokes' lines vary in a
similar manner.
APPLICATION OF PURE
ROTATIONAL RAMAN SPECTRA
• From the pure rotational Raman spectra,
noting the separation between the lines,
the value of B can be obtained from which
the moment of inertia and the bond
length of the diatomic molecules can be
calculated
ROTATIONAL-VIBRATIONAL
RAMAN SPECTRA OF
DIATOMIC MOLECULES
• For large molecules, the lines obtained due to
rotational transitions are so weak that they are
beyond resolution. Hence we have pure
vibrational Raman spectra for which the
selection rules are same as for pure vibrational
spectra i.e. Δ v = + 1, + 2, ….. However in
case of diatomic gaseous molecules, the
resolution of rotational fine structure is sufficient
and can be studied. Thus diatomic gaseous
molecules give rotational-vibrational Raman
spectra
ADVANTAGES OF RAMAN
SPECTROSCOPY OVER INFRARED SPECTROSCOPY
• Raman frequencies are independent of
the frequency of the incident radiation,
hence by suitable adjusting the frequency
of the incident radiation, Raman spectra
can be obtained in the visible spectrum
• Raman spectra can be obtained even for
molecules such as O2' N2' C12 etc. which
have no permanent dipole moment. Such
a study has not been possible by infra-red
spectroscopy.
• Raman spectra can be obtained not only
for gases but even for liquids and solids
whereas infra-red spectra for liquids and
solids are quite diffuse.
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