RAMAN Spectroscopy
Introduction
Raman spectroscopy is a non-destructive spectroscopic
analysis that offers comprehensive data on chemical structure, phase and polymorphic
state, crystallinity, and molecular interactions. It is based on how light
interacts with chemical bonding in a substance. The vibrational, rotational,
and low-frequency modes of molecules are studied using this technique.
Raman scattering, also known as inelastic photon
scattering, is the basis of Raman spectroscopy. Although X-rays can also be
employed, monochromatic light is often produced by lasers in the visible, near
infrared, or near ultraviolet spectrum. The
energy of the laser photons is pushed up or down as a result of the laser
light's interactions with phonons, molecular vibrations, or other excitations
in the system. The energy shifts reveals details about the system's vibrational
modes.
Although rotational and other low-frequency modes of
systems may also be seen, Raman spectroscopy (/ˈrɑːmən/), which bears the name
of the Indian Physicist C. V. Raman, is primarily used to discern the
vibrational modes of molecules. Raman spectroscopy is frequently used in
chemistry to give compounds a unique structural fingerprint.
The Raman effect depends upon the interaction of an
electron cloud within a sample and of the external electric field of
monochromatic light. Depending on the polarizability of the molecule, this
interaction might cause a dipole moment within the molecule. There cannot
actually be a change in energy level because the laser light does not stimulate
(excite) the molecule to other levels. The emission that occur in fluorescence
or phosphorescence of a molecule in an excited electronic state emitting a
photon and returning to the ground electronic state, frequently to a
vibrationally excited state on the ground electronic state potential energy
surface, should not be confused with the Raman effect.
History
The Indian Scientist C. V. Raman, who discovered the
Raman effect in 1928 along with K. S. Krishnan and separately by Grigory
Landsberg and Leonid Mandelstam in inorganic crystals, was one of its
discoverers. For this discovery, Raman received the Nobel Prize in Physics in
1930. Franco Rasetti made the initial detection of Raman spectra in gases in
1929.
Adolf Smekal predicted the inelastic scattering of
light in 1923, but it wasn't until 1928 that it was actually observed. George
Placzek, a Czechoslovak physicist, created a comprehensive, ground-breaking
theory of the Raman phenomenon between 1930 and 1934. With photographic and
then spectrophotometric detection, the mercury arc evolved into the main source
of illumination.
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| RAMAN Instrument |
Principle
Raman spectroscopy works on the basis of the idea that
when monochromatic light passes through a sample, it may be reflected,
absorbed, or scattered. The vibration and rotational properties of the
scattered photons cause them to have a different frequency from the incident
photon. The IR spectra are used to study the change in wavelength that arises
from this.
- The degree of freedom in Raman's scattering can be calculated using the 3N formula.
- The different types of Raman's Spectroscopy include Non-linear Raman Spectroscopic, Resonance Raman Spectroscopy (RRS), Surface-enhanced Raman Spectroscopy (SERS), and Micro-Raman Spectroscopy techniques.
Raman scattering
Scattering of photons by the excited molecules that are
at higher energy levels is known as Raman Scattering.
Rayleigh
scattering
Rayleigh scattering is the most prevalent transition
because it involves no change in the molecule's vibrational state.
Stokes lines
As the frequency of scattered light is lower than the
frequency of incident light, Stokes lines form in Raman spectra.
Anti-Stokes lines
The frequency of scattered light is greater than the
frequency of incident light, Anti Stokes lines form in Raman spectra. The
anti-Stokes transition is the least common because it involves the molecule to
be vibrationally excited before the photon strikes it.
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| Scattering Types |
Instrumentation
One or more single-colored light sources, lenses, and
filters are used in a Raman spectrometer to focus the light and distinguish
between dispersed and reflected light, respectively. To separate the light into
its component parts and to detect weak light, a prism is utilised. To examine
the data, the spectrum is afterwards acquired and displayed on the monitor. A
mercury lamp and photographic plates were employed in the past to record
spectra; today, lasers are used as excitation light sources. Raman and Krishnan
utilised a mercury lamp and photographic plates to capture spectra because it
took more than three decades for lasers to become widely accessible. Due to the weak light sources, low sensitivity of the
detectors, and low Raman scattering cross-sections of the majority of
materials, early spectra required hours or even days to capture. The
photographic spectra were still dominated by a broad center line due to
Rayleigh scattering of the excitation source, despite the employment of several
coloured filters and chemical solutions to choose specific wavelength regions
for excitation and detection. Particularly during the 1980s, technological
advancements have greatly increased the sensitivity of Raman spectroscopy.
Charge-coupled devices are now the most used kind of modern detectors (CCDs).
Prior to the use of CCDs, photodiode arrays and photomultiplier tubes were
widely used.
Types of Samples
Powders, Solids, liquids, slurries, gels, gases, Inorganic,
biological materials, organic and Pure chemicals, mixtures and solutions, metallic
oxides and corrosion. In common, RAMAN Spectrometer is not used for the analysis
of metals & their alloys.
Lasers
A laser or other light source is necessary for Raman
spectroscopy. The bandwidth of the laser source employed determines the spectrum's
resolution. Due to the increase in Raman scattering cross-sections, shorter
wavelength lasers typically produce stronger Raman scattering, however problems
with sample deterioration or fluorescence may occur.
Although pulsed lasers can also be utilised,
continuous wave lasers are more frequently used for standard Raman
spectroscopy. They are frequently more effective for various types of Raman
spectroscopy, including transient, time-resolved, and resonance Raman, despite
having broader bandwidths than their CW counterparts.
Detectors
Typically, Raman scattered light is collected, either
distributed by a spectrograph or utilised in conjunction with an interferometer
for Fourier Transform (FT) methods of detection. Many times, FT-IR
spectrometers that are now on the market can be converted into FT-Raman
spectrometers.
- Dispersive Raman Detectors
The majority of contemporary Raman spectrometers use
array detectors like CCDs. Existing CCDs come in a variety of designs that are tailored
to work with various wavelength ranges. For very weak signals and/or pulsed
lasers, intensified CCDs can be employed. The CCD's size and the spectrograph's
focal length have an impact on the spectral range. Monochromators connected to
photomultiplier tubes were once widely used. To scan through a spectral range,
the monochromator would need to be moved in this case.
- FT–Raman Detectors
NIR lasers are nearly always utilized with FT-Raman,
and depending on the exciting wavelength, the proper detectors must be used.
Detectors made of germanium or indium gallium arsenide are frequently employed.
Filters
In order to get good quality Raman spectra, a laser
rejection filter is typically required to separate the Raman scattered light
from the Rayleigh signal and reflected laser signal. Usually, optical filters
with notches or long passes are employed for this. Prior to the invention of
holographic filters, it was typical to isolate the desired signal using a
triple-grating monochromator in subtractive mode. While holographic filters
often reflect some of the low frequency bands in addition to the unshifted
laser light, this can still be utilised to record very minor Raman shifts. Nevertheless,
volume hologram filters, which enable shifts as small as 5/cm, are becoming
more widely used.
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| Instrumentation of RAMAN spectroscopy |
Applications
One form of vibrational spectroscopy that necessitates
a solid knowledge of light's physical characteristics is Raman spectroscopy.
Every time unidentified materials need to be identified, this method is widely
utilized since it gives a chemical "fingerprint" of the substance
being tested.
Raman spectroscopy examines a material's chemical
composition and yields data on polymorphism, phase internal tension or stress, chemical
identity and structure, Uncleanliness, and contamination. A Raman spectrum
typically serves as a unique chemical fingerprint for a certain molecule or
material and can be used to swiftly identify the substance in question or set
it apart from similar substances. Raman
spectroscopy has a spatial resolution of between 0.5 and 1 m, making it
suitable for microscopic examination. With a Raman microscope, this kind of
analysis is achievable.
Difference between RAMAN and IR Spectroscopy
The most common method for identifying unknown organic
compounds is FT-IR. Although Raman spectroscopy also offers information on
molecular bonding, it does so in the areas where IR is least helpful. The two
methods work well together and enable the best identification of the unknown.
The ability to distinguish between various crystal forms of a given substance
is a special strength. We are able to obtain data from below the surface and
determine depth profiles by fusing this spectrometer with a confocal
microscope.
There are some major, fundamental differences between
FTIR and Raman spectroscopy. Whereas IR spectroscopy depends on a change in the
dipole moment, Raman spectroscopy depends on a change in the polarizability of
a molecule.
Raman spectroscopy, as opposed to IR spectroscopy,
measures the relative frequencies at which a sample scatters light rather than
the absolute frequencies at which a sample absorbs light. FTIR spectroscopy can
detect polar bonds and hetero-nuclear functional group vibrations, particularly
OH stretching in water. Raman, on the other hand, can detect homo-nuclear
molecular bonds. It can distinguish between C-C & C=C bonds, as an example.
Fourier transform infrared spectroscopy (FTIR) is a
type of vibrational spectroscopy that is based on the absorbance,
transmittance, or reflectance of infrared light. Light is absorbed in varying
amounts in a sample at specific frequencies that match the vibrational
frequencies of the sample's bonds using this technique.
Both of these methods work with microscopic
techniques. Raman spectroscopy has the advantage of requiring little to no
sample preparation, while the FTIR method has sample thickness, homogeneity,
and dilution limitations to avoid saturation. The distinction between the two
methods in dealing with interference is the main advantage of FTIR.
Fluorescence may impair the ability to obtain Raman spectra, whereas FTIR does
not.
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IR Spectroscopy |
RAMAN Spectroscopy |
|
Molecules absorb
IR radiations resulting in different types of transitions. |
Molecules absorb
any kind of light that results in the scattering of light. |
|
Many molecules
can absorb IR light. |
Only one
molecule shows scattering out of a million molecules. |
|
Any type of
sample can be used like solid, liquid, solutions, etc. |
Gaseous samples give
better results in the RAMAN spectrometer. |
|
An increase or
decrease in temperature changes the absorbance of IR light. |
At any
temperature, the stokes shift is more prominent but the anti-stokes shift is
not prominent so, when the temperature increase anti-stokes shift becomes
prominent. |
|
IR active
molecules are those that have dipole moments. |
RAMAN active
molecules have dipole moments as well as polarizability power. |
|
IR active
molecules would also RAMAN active molecules. |
RAMAN active
molecules are not always IR active |
|
O=C=O Dipole moment
zero IR inactive |
O=C=O Dipole moment
zero RAMAN active, due to polarization. |
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