Raman spectrascopy
Raman spectroscopy is a spectroscopic technique used in condensed
matter physics and chemistry to study vibrational, rotational, and other lowfrequency modes in a system.
Raman scattering is named after Indian
physicist C. V. Raman who discovered it
in 1928. For his observation of this effect
Raman was awarded the 1930 Nobel
Prize in Physics. This was and remains
the shortest time from a discovery to
awarding of the Prize.
Raman spectroscopy relies on inelastic scattering, or Raman scattering of
monochromatic light by molecules, usually from a laser in the visible, near
infrared, or near ultraviolet range.
1
Raman scattering
Raman scattering or the Raman effect is the inelastic scattering of a photon
Rayleigh scattering
•Most light passing through a transparent substance undergoes Rayleigh
scattering.
•This is an elastic effect, which means that the light does not gain or lose
energy during the scattering. Therefore it stays at the same wavelength.
•The amount of scattering is strongly dependent on the wavelength, being
proportional to λ-4.
However, a small fraction of the scattered light (approximately 1 in 1
million photons) is scattered by an excitation, with the scattered
photons having a frequency different from, and usually lower than,
the frequency of the incident photons.
Rayleigh scattering (named after Lord Rayleigh) is the scattering of light
or other electromagnetic radiation by particles much smaller than the
wavelength of the light. It can occur when light travels in transparent solids
and liquids, but is most prominently seen in gases.
Why is the sky blue?
The amount of scattering is
strongly dependent on the
wavelength, being proportional to
λ-4:
Figure showing the more intense
scattering of blue light by the
atmosphere relative to red light
2
Raman scattering
1. In a gas, Raman scattering can occur with a change in vibrational,
rotational or electronic energy of a molecule.
2. Raman spectroscopy is commonly used in chemistry, since vibrational
information is very specific for the chemical bonds in molecules.
3. It therefore provides a fingerprint by which the molecule can be
identified. The fingerprint region of organic molecules is in the range
500-2000 cm-1.
4. Another way that the technique is used is to study changes in chemical
bonding, e.g. when a substrate is added to an enzyme.
Vibrational, rotational or electronic energy of a molecule
3
In Rayleigh scattering
1. A photon interacts with a molecule, polarizing the electron cloud and raising
it to a “virtual” energy state.
2. This is extremely short lived (on the order of 10-14 seconds) and the
molecule soon drops back down to its ground state, releasing a photon.
3. This can be released in any direction, resulting in scattering.
4. However since the molecule is dropping back to the same state it started in,
the energy released in the photon must be the same as the energy from the
initial photon. Therefore the scattered light has the same wavelength.
Basic theory
The laser light interacts with
phonons or other excitations
in the system, resulting in
the energy of the laser
photons being shifted up or
down. The shift in energy
gives information about the
phonon modes in the
system.
Infrared spectroscopy yields
similar, but complementary
information.
Energy level diagram showing the states
involved in Raman signal. The line thickness is
roughly proportional to the signal strength from the different
transitions.
Wavelengths close to the
laser line, due to elastic
Rayleigh scattering, are
filtered out while the rest of
the collected light is
dispersed onto a detector.
4
Raman effect
The incident photon excites one of the electrons into a virtual state.
For the spontaneous Raman effect, the molecule will be excited from the ground
state to a virtual energy state, and relax into a vibrational excited state, which
generates Stokes Raman scattering.
If the molecule is in a vibrational state to begin with and after scattering is in its
ground state then the scattered photon has more energy, and therefore a shorter
wavelength. This is called anti-Stokes scattering.
A molecular polarizability change, or amount of deformation of the electron cloud,
with respect to the vibrational coordinate is required for the molecule to exhibit the
Raman effect.
The amount of the polarizability change will determine the Raman scattering
intensity, whereas the Raman shift is equal to the vibrational level that is involved.
5
Normally in Raman spectroscopy only the Stokes half of the spectrum is
used, due to its greater intensity.
6
7
In one of Raman’s experiments demonstrating inelastic scattering he used light
from the Sun focused using a telescope to obtain a high intensity light.
This was passed through a monochromatic filter, and then through a variety of
liquids where it underwent scattering.
After passing through these he observed it with a crossed filter that blocked the
monochromatic light. Some light was seen passing through this filter, which
showed that its wavelength had been changed.
8
http://www.doitpoms.ac.uk/tlplib/raman/comparison.php
9
A simplified diagram of a Raman spectrometer’s operation is shown below:
spectral resolution
1. An important consideration in Raman spectroscopy is the spectral
resolution, the ability to resolve features within the spectrum.
2. Two ways to increase spectral resolution,
by increasing the focal length
by changing the grating used to disperse the spectrum.
http://www.doitpoms.ac.uk/tlplib/raman/method.php
10
Raman microspectroscopy
Raman spectroscopy can also be used for microscopic analysis and imaging.
Direct imaging and hyperspectral imaging (chemical imaging).
The instrument in the Cambridge Department of Materials Science is a typical
microspectrometer, manufactured by Renishaw. An interactive diagram of this is
shown below
http://www.doitpoms.ac.uk/tlplib/raman/raman_microspectroscopy.php
Raman spectra of two methyl chlorosilanes
illustrating how the two species can be distinguished by
Raman spectroscopy. Characteristic Raman bands of
each species are marked with an asterisk (*).
11
Raman spectrascopy
Advantages and disadvantages
Advantages...
1. Can be used with solids, liquids or gases.
2. No sample preparation needed.
3. Spectra from each material are unique, can be used to identify materials
conclusively.
4. Non-destructive
5. No vacuum needed unlike some techniques, which saves on expensive
vacuum equipment.
6. Short time scale. Raman spectra can be acquired quickly.
7. Can work with aqueous solutions (infrared spectroscopy has trouble with
aqueous solutions because the water interferes strongly with the wavelengths
used)
8. Glass vials can be used (unlike in infrared spectroscopy, where the glass
causes interference)
9. Can use down fibre optic cables for remote sampling.
10. Fast analyses
Disadvantages:
1. Cannot be used for metals or alloys.
2. The Raman effect is very weak, which leads to low sensitivity, making it difficult
to measure low concentrations of a substance. This can be countered by using
one of the alternative techniques (e.g. Resonance Raman) which increases the
effect.
3. Can be swamped by fluorescence from some materials.
4. Samples with color may absorb laser light and burn.
5. Expensive
12
Raman and IR
13
Raman active modes
For a mode to be Raman active it must involve a change in the polarisability, α
of the molecule i.e.
where q is the normal coordinate and e the equilibrium position.
This is known as spectroscopic selection. Some vibrational modes
(phonons) can cause this. These are generally the most important,
although electronic modes can have an effect, and rotational modes may
be observed in gases at low pressure.
The spectroscopic selection rule for infrared spectroscopy (IR) is that
only transitions that cause a change in dipole moment can be observed.
Because this relates to different vibrational transitions than in Raman
spectroscopy, the two techniques are complementary. In fact for
centrosymmetric (centre of symmetry ) molecules the Raman active
modes are IR inactive, and vice versa. This is called the rule of mutual
exclusion.
14
Raman active modes
http://www.doitpoms.ac.uk/tlplib/raman/active_modes.php
15
Molecular Dipole Moments
Even though the total charge on a molecule is zero, the nature of
chemical bonds is such that the positive and negative charges do
not completely overlap in most molecules. Such molecules are
said to be polar because they possess a permanent dipole
moment.
A good example is the dipole moment of the water molecule.
Molecules with mirror symmetry like oxygen, nitrogen, carbon
dioxide, and carbon tetrachloride have no permanent dipole
moments. Even if there is no permanent dipole moment, it is
possible to induce a dipole moment by the application of an
external electric field. This is called polarization and the
magnitude of the dipole moment induced is a measure of the
polarizability of the molecular species.
Dipole Moment of Water
The asymmetry of the water molecule leads to a dipole moment in the
symmetry plane pointed toward the more positive hydrogen atoms. The
measured magnitude of this dipole moment is
P = 6.2x10-30 C.m
Treating this system like a negative charge of 10 electrons and a positive
charge of 10e, the effective separation of the negative and positive charge
centers is
D = P/10e = 3.9 x 10-12 m
This is 0.0039 nm compares with about .05 nm for the first Bohr radius of a
hydrogen atom and about .15 nm for the effective radius of hydrogen in liquid
form, so the charge separation is small compared to an atomic radius.
The polar nature of water molecules allows them to bond to each other in
groups and is associated with the high surface tension of water.
16
Dipolar Bonding in Water
The dipolar interaction between water
molecules represents a large amount of
internal energy and is a factor in water's large
specific heat. The dipole moment of water
provides a "handle" for interaction with
microwave electric fields in a microwave
oven. Microwaves can add energy to the
water molecules, whereas molecules with no
dipole moment would be unaffected.
The polar nature of water molecules allows
them to bond to each other in groups and is
associated with the high surface tension of
water. The polar nature of the water molecule
has many implications. It causes water vapor
at sufficient vapor pressure to depart from the
ideal gas law because of dipole-dipole
attractions. This can lead to condensation
and phenomena like cloud formation, fog, the
dewpoint, etc. It also has a great deal to do
with the function of water as the solvent of life
in biological systems.
Polarizability is the relative tendency of a charge distribution, like the electron
cloud of an atom or molecule, to be distorted from its normal shape by an external
electric field, which may be caused by the presence of a nearby ion or dipole.
The electronic polarizability α is defined as the ratio of the induced dipole moment P
of an atom to the electric field E that produces this dipole moment.
P = αE
Polarizability has the SI units of C·m2·V-1 = A2·s4·kg-1 but is more often expressed as
polarizabilty volume with units of cm3 or in Å3 = 10-24 cm3.
The polarizability of individual particles is related to the average electric susceptibility
of the medium by the Clausius-Mossotti relation.
Note that the polarizability α as defined above is a scalar quantity. This implies that
the applied electric fields can only produce polarization components parallel to the
field. For example, an electric field in the x-direction can only produce an x
component in . However, it can happen that an electric field in the x-direction,
produces a y or z component in the vector P . In this case α is described as a tensor
of rank 2, which is represented with respect to a given system of axes (frame of
reference) by a 3x3 matrix.
17
18
Applications
Measuring/mapping stress
Raman spectroscopy can be used to measure stress and strain in materials.
Tensile strain increases the length of the bonds and the tension in them,
hence changing the frequency of the phonons. It therefore causes a shift in
the observed Raman bands towards lower wavenumbers.
Forensics, explosives/drugs detection
Photo of Raman integrated tunable sensor
Advances in technology have led to much smaller spectrometers, which are
moving from the laboratory bench towards handheld devices that can be used for
analysis in the field. They may be linked to a library of spectra, and can be used
by law enforcement and customs officials to detect explosives, drugs and other
chemicals. They are also useful for quickly identifying possibly hazardous
materials e.g. after a spillage.
Pictured is a Raman integrated tunable sensor (RAMITS) developed by the US
government. It has a probe coated with silver nanoparticles, which allow Surface
Enhanced Raman Spectroscopy, boosting the signal. The instrument is handheld
and battery powered.
Process monitoring
Raman spectroscopy is a non-destructive process, and can
be used to monitor industrial processes. The speed of
analysis means that it can give almost real-time information.
Another advantage is that the light to be monitored can be
sent down fibre-optics, so that the Raman equipment can be
located some distance away from the actual processing.
Uncovering artistic techniques
As well as monitoring state of the art processes, Raman
spectroscopy is being used to uncover the secrets of
ancient artefacts. Scientists at Trinity College in Dublin are
using Raman spectroscopy to examine the famous Book of
Kells, an illustrated manuscript dating from the 9th century.
They hope to determine the composition and origins of the
paper, inks and pigments used, which will tell them about
techniques used and trade routes of the age.
A page from the book of Kells
19
Life on Mars
Raman spectroscopy could also be used to search for life on Mars. Modern
Raman technology has been miniaturised to the point that a small spectroscope
will be carried on a future mission to the planet. The instrument will be used to
look for evidence of life and/or life supporting conditions either in the present or
the distant past, as well as more general analysis of the Martian surface. Similar
instruments could be featured on missions to other potential sites of life such as
Europa or Callisto.
Carbon nanotubes
Because of their structure, carbon nanotubes can be made to resonate with light.
They may resonate with either the incident wavelength, or Raman scattered
wavelengths. Resonance can also occur for a number of different modes. Some of
the most important are the radial breathing mode, the disorder mode and the high
energy mode.
Observations of these can be used to determine important properties of the
nanotubes, such as their diameter and strain. Raman spectroscopy is one of the
easiest ways of measuring these vital properties.
Going further
Books
Cardona M, Light Scattering in Solids (Topics in Applied Physics Volume 8),
Springer-Verlag, 1975.
Websites
The Nobel Prize in Physics 1930 - including a biography of Raman, the 1930
Nobel Prize presentation speech and his Nobel lecture.
Sir C. V. Raman and the story of the Nobel prize- an in depth look at Raman’s
1930 prize.
Theory of Raman Spectroscopy– a guide to the cause of Raman scattering.
Assigning Spectra - an interactive tutorial on using group theory to assign spectra.
Raman spectroscopy: a complex technology moving from lab to the clinic — and
before too long, the marketplace – examples of Raman spectroscopy applications.
20
21