Download Raman Spectroscopy

Raman
Spectroscopy
BY
Dr. Bhawna
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.
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.
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 transitions is called ``absorption spectrum.``
• 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. The energy absorbed for any transition is equal to the
difference in the energies of the two levels involved. It is found that
these energies for transition are in the order
• 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. The different types of molecular
spectra generally observed, depending upon the energies absorbed
and the regions of the electromagnetic spectrum in which they are
observed, are shown in Fig. and are briefly explained below:
(1) Pure rotational (Microwave)
spectra.
• If the energy absorbed by the molelcule 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.
(2) 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.
•
(3) 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 for a given electronic transition, a set of
bands is observed. This set of bands is called a band group or a
band system. Each electronic transition gives a band system. The
complete set of band systems obtained due to different electronic
transitions gives the electronic band spectrum of the gaseous
molecule. 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).
(4) 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). The method is based
upon applying such frequencies on the sample
so that it is in resonance with the applied
frequency.
(5) 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).
Frequencies of this range are applied on
the sample to bring the sample in
resonance condition.
(6) Raman spectra.
• This is also a type of vibrational-rotational spectrum but is based on
scattering of radiation and not on the absorption of radiation by the
sample. It is based upon the principle that when a sample is hit by
monochromatic radiation of the visible region and scattering is
observed at right angles to the direction of the incident beam, the
scattered radiation have frequency equal to that of the incident
beam (called Rayleigh scattering) as well as frequencies different
(higher as well as lower) than that of the incident beam (called
Raman scattering). The difference in the frequencies of the incident
beam and that of the scattered beam (called Raman frequencies)
are similar to those needed for the vibrational and rotational
transitions. However, by suitably adjusting the frequency of the
incident radiation, Raman spectra are observed in the visible region
(12,500-25,000 cm-1).
RAMAN SPECTRA
GENERAL INTRODUCTION
• It is a special type of spectroscopy which deals not with
the absorption of electromagnetic radiation but deals
with the scattering of light by the molecules. It is
observed that 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.
• Prof. C.V.Raman of Calcutta University, however,
observed in 1928 that when a substance (gaseous, liquid
or solid) 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.
• Thus the Raman frequencies observed for
a particular substance are characteristic of
that substance. The various observations
thus made by Raman constitute what is
called Raman effect and the spectrum
observed is called Raman spectrum.
Thus in a simple way, Raman spectrum
may be represented as shown in Fig.
EXPLANATION FOR OBSERVING
RAYLEIGH LINE AND RAMAN
LINES
• When the incident photon hits the molecule, this collision
may be elastic or inelastic. Elastic collision means that
the colliding particles will return with the same energy.
Thus the scattered photon has the same frequency as
that of the incident photon. This explains Rayleigh
scattering or Rayleigh line. However, if the collision is
inelastic, still the law of conservation of energy will hold
good i.e. total energy before collision and after collision
must remain the same. Thus some energy may be
transferred from the incident photon to the molecule so
that the scattered photon has lower energy and hence
lower frequency than that of the incident photon.
• This explains the occurrence of Stocks' lines.
Alternatively, some energy may be transferred from the
incident photon to the molecule so that the scattered
photon has higher energy and hence higher frequency
than that of the incident light. This explains of the
occurrence of anti-Stoke's lines. This explanation may
be further elaborated as follows:
• When the photon strikes the molecule, the energy is
absorbed by the molecule and it gets excited to some
higher energy level. Now if it returns to the original level,
it will emit the same energy as absorbed and thus we
have Rayleigh scattering. However, in most of the
cases, the excited molecule does not return to the
original level. It may return to a level higher than the
original level thereby emitting less energy than
absorbed.
• This explains the occurrence of Stokes'
lines. Thus a part of the energy of the
incident photon remains absorbed by the
molecule (so that molecule has higher
energy than before). Alternatively, the
excited molecule may return to a level
lower than the original level. Thus more
energy is emitted than absorbed. This
explains the occurrence of anti-Stokes'
lines. In this case, the molecule has less
energy than before. The different cases
may be represented diagrammatically as
shown in Fig.
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. We write
• μ= αE
• where the quantity depends upon the nature of
the molecules* and is called polarizability of the
molecule. Thus polarizability is the ratio of the
induced dipole moment to the strength of the
electric field.
•
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. For example,
in case of H2 molecule, the distortion
produced is more when the electric field is
applied parallel to the bond than when it is
applied perpendicular to it and we write
α11>α1
• 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 he orientation of the
molecule with respect to the direction of the electric field.
Hence all diatomic molecules, linear molecules and nonspherical 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).
• In fact, this is one of the reasons for
importance of rotational Raman
spectroscopy over infrared spectroscopy
because the latter requires that the
molecule must have a permanent dipole
moment whereas the former only requires
that polarizability of the molecule must
change during vibration.
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, as explained earlier.
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 which can
be explained as follows:
ADVANTAGES OF RAMAN
SPECTROSCOPY OVER INFRARED SPECTROSCOPY
• Raman spectroscopy has a number of
advantages over infra-red spectroscopy as
briefly explained below:
•
(i) Since 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
range where they can be easily observed rather
than the more difficult infra-red range.
• (ii) 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.
•
(iii) 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.
• Because of the ease with which the
Raman spectra can be studies, the
molecular structure of a large number of
compounds have been determined by
Raman spectroscopy. However infra-red
method still continues to remain popular.
THANKS