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Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in a system.

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.

Raman spectrascopy

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.

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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.

Raman scattering

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 lightor 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?

Figure showing the more intense scattering of blue light by the atmosphere relative to red light

The amount of scattering is strongly dependent on the wavelength, being proportional to λ-4:

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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 vibrationalinformation 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.

Raman scattering

Vibrational, rotational or electronic energy of a molecule

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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

Energy level diagram showing the states involved in Raman signal. The line thickness is roughly proportional to the signal strength from the different transitions.

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.

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.

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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.

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Normally in Raman spectroscopy only the Stokes half of the spectrum is used, due to its greater intensity.

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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.

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http://www.doitpoms.ac.uk/tlplib/raman/comparison.php

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A simplified diagram of a Raman spectrometer’s operation is shown below:

1. An important consideration in Raman spectroscopy is the spectralresolution, 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.

spectral resolution

http://www.doitpoms.ac.uk/tlplib/raman/method.php

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Raman microspectroscopy

http://www.doitpoms.ac.uk/tlplib/raman/raman_microspectroscopy.php

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

Raman spectra of two methyl chlorosilanesillustrating how the two species can be distinguished by Raman spectroscopy. Characteristic Raman bands of each species are marked with an asterisk (*).

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Raman spectrascopyAdvantages 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

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Raman and IR

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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.

For a mode to be Raman active it must involve a change in the polarisability, αof the molecule i.e.

Raman active modes

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Raman active modes

http://www.doitpoms.ac.uk/tlplib/raman/active_modes.php

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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 donot 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 WaterThe 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 liquidform, 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.

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Dipolar Bonding in WaterThe 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 lifein 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 = αEPolarizability 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 susceptibilityof 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 xcomponent 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 tensorof rank 2, which is represented with respect to a given system of axes (frame of reference) by a 3x3 matrix.

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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 detectionPhoto of Raman integrated tunable sensorAdvances 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

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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

BooksCardona M, Light Scattering in Solids (Topics in Applied Physics Volume 8), Springer-Verlag, 1975.

WebsitesThe 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.

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