Raman spectroscopy is a phenomenon of inelastic scattering of light which first

postulated by Smekal in 1923 and first observed experimentally in 1928 by Raman and

Krishnan. Vibrational and rotational transition can be explored by examining the

frequencies present in the radiation scattered by molecules in Raman Spectroscopy. Two

types of scattering are readily identified. The most intense form of scattering, Rayleigh

scattering (component of radiation scattered without change of frequency), occurs when the

electron cloud relaxes without any nuclear movement. This is essentially an elastic process

and there is no appreciable change in energy. Raman scattering on the other hand is a much

rarer event which involves only one in 106-108 of the photons scattered. This occurs when the

light and the electrons interact and the nuclei begin to move at the same time. Since the

nuclei are much heavier than the electrons, there is an appreciable change in energy of the

molecule to either lower or higher energy depending on whether the process starts with a

molecule in the ground state (Stokes scattering) or from the molecule in a vibrationally

excited state (anti- stokes scattering). Most molecules at rest prior to interaction with the

laser and at room temperature are likely to be in the ground vibrational state. Therefore the

majority of Raman scattering will be Stokes Raman scattering. The ratio of the intensities of

the Stokes and anti-Stokes scattering is dependent on the number of molecules in the

ground and excited vibrational levels. This can be calculated from the Boltzmann equation.

The gross selection rule for rotational Raman transitions is that the molecule must

be anisotropically polarizable. The gross selection rule for vibrational Raman transitions

is that the polarizability should change as the molecule vibrates. Raman scattering arises

from a change in polarizability in the molecule. This means that symmetric vibrations will give

the most intense Raman scattering. This is in complete contrast to infrared absorption

where a dipole change in the molecule gives intensity and, at a very simple level, this means

asymmetric rather than symmetric rather than symmetric vibrations will be intense.

One crucial result which arises from this analysis is that irrespective of other

symmetry considerations, for a centrosymmetric molecule, only vibrations which are g in

character can be Raman active and only vibrations which are u in character can be infrared

active. This is because irrespective of the exact irreducible representation, the g and u labels

can be multiplied out and the final product must contain the totally symmetric representation

and hence g. The rules are g x g=g, u x u=g and g x u=u. Since the Raman operators are g

in character and the ground state is g, the excited state must be g if the vibration is to be

allowed. In contrast, the infrared operator is u in character and so the excited state must be u

if the vibration is to be allowed. Thus in a molecule with a centre of symmetry, vibrations

which are Raman active will not be infrared active and vibrations which are infrared active

will not be Raman active. Note that, the symmetric vibrations (g) which are Raman active,

and the asymmetric vibrations (u) which are infrared active. This analysis leads to a rule

known as the mutual exclusion rule, which states that any vibration in a molecule

containing a centre of symmetry can be either Raman or infrared active, but not both. In

molecules without a centre of symmetry, there is no such specific rule. Nonetheless, in

general, symmetric vibrations are more intense in Raman scattering and asymmetric

vibrations in infrared scattering.

Comparisons between Raman and Infrared spectroscopy, Raman spectroscopy is

scattering spectroscopy which are involves the carbon allotropes and polarizable bond: C-C,

C=C. Scattering involves a momentary distortion of the electrons distributed around a

molecular bond. Thus, the molecule is temporarily polarized (a momentarily induced dipole

that disappears upon relaxation and reemission. Infrared spectroscopy is absorption

spectroscopy, which are involves the functional groups and polar bonds: C=O, O-H. It

requires the vibrational mode of the molecule to have a change in the dipole moment or

charge distribution associated with it. Only then, a radiation of same frequency interacts with

the molecule, and promotes it to the excited state. To be visible in Raman vibration must

change the polarizability of the molecule. Whereas, to be visible in Infrared vibration, must

change the dipole moment of the molecule. An important advantage of Raman spectra over

infrared lies in the fact that water does not cause interference; indeed, Raman spectra can

be obtained from aqueous solution. Furthermore, glass or quartz cells can be employed, thus

avoiding the inconvenience of working with sodium chloride or other atmospherically

unstable confinements. Thus aqueous solutions can be studied by Raman spectroscopy but

not by Infrared. For frequency range, Raman is from 4000- 50 cm-1 while Infrared from 4000-

700cm -1.

Raman instrumentation was developed (based around arc lamps and photographic

plates) and soon became very popular up until the 1950's.Since these early days, Raman

instrumentation has evolved markedly. Modern instrumentation typically consists of a

laser, sample illumination system, wavelength selector (Rayleigh filter), and a detector

(typically a CCD or ICCD). Raman spectroscopy is essentially emission spectroscopy, and

the bulk of the instrumentation is simply a typical visible-region spectrometer; the

distinguishing characteristic of Raman work, of course, is the exciting source. The advent of

accessible and relatively inexpensive laser sources during the past few years has caused a

minor revolution in Raman techniques, by largely displacing the traditional mercury

discharge lamp as an exciting source. Formerly the process of obtaining a good Raman

spectrum of anything but the most straightforward samples involved as much art as science,

required 10-20 ml of sample, and was often a very time-consuming operation; now Raman

spectra of virtually all samples can be run on a completely routine basis using one milliliter or

less of sample and taking a few minutes only. In fact the laser is almost ideal as a Raman

source; it gives a very narrow, highly monochromatic beam of radiation, which may be

focused very finely into a small sample, and which packs a relatively large power. In

conventional Raman experiments the sample is illuminated by monochromatic light. The

registration of low intensity Raman scattering in the presence of strong Tyndall and Rayleigh

scattering implies special requirements for Raman spectrometers. A Raman spectrometer

has to combine very good filter characteristics for eliminating Rayleigh and Tyndall

scattering with high sensitivity for detecting very weak Raman bands. There are three types

of Raman instruments which are currently available; those are Raman grating spectrometer

with single channel detector, FT- Raman spectrometer with near infrared excitation, and

Raman grating polychromator with multichannel detector. Raman signal is normally quite

weak and people are constantly improving Raman spectroscopy techniques. Many different

ways of sample preparation, sample illumination or scattered light detection were invented to

enhance intensity of Raman signal, stimulated raman, where the sample was irradiated with

a very strong laser pulse. CARS (Coherent Anti-Stokes Raman), two very strong collinear

lasers irradiate a sample. Resonance Raman, the Resonance Raman Effect takes place

when the excitation laser frequency is chosen in a way that it crosses frequencies of

electronic excited states and resonates with them.

The classical theory of the Raman Effect, although not adequate, is worth

consideration since it leads to an understanding of a concept basic to this form of

spectroscopy is the polarizability of a molecule. When a molecule is put into a static electric

field it suffers some distortion, the positively charged nuclei being attracted towards the

negative pole of the field, the electrons to the positive pole. This separation of charge centers

causes an induced electric dipole moment to be set up in the molecule and the molecule is

said to be polarized. The size of the induced dipole, μ, depends both on the magnitude of the

applied field, E, and on the ease with which the molecule can be distorted. Quantum theory

of Raman Effect, as a stream of photons collides with a particular molecule the photons will

be deflected without change in energy if collisions are perfectly elastic. If energy is

exchanged between photon and molecule, the collision is said to be inelastic. The molecule

can gain or lose discrete amounts of energy in accordance with quantal laws; the energy

must coincide with a transition between two molecular energy levels.