One of the important tools that we utlize in our research is "Raman Scattering". Here is a simple explanation to
understand the process of Raman scattering.
When the cue-ball hits an object-ball in the game of billiards, the cue-ball may stop or it may continue moving but with a
different speed and/or direction. This collision between the two balls that follow the rules of momentum conservation
can be categorized as either elastic or inelastic, depending upon the conservation of kinetic energy. In an almost similar way,
when a photon (light) collides with an electron (or atom/molecule) in a material, it can go through an elastic or an inelastic
process. While the object-balls in billiards are nearly free to move away after the collision, the electrons are bound to the
atoms, the atoms are bound to the molecules and the molecules are bound to the lattice with comparatively much larger forces,
and hence it is only the photon that keeps moving before and after the collision. This process is known as the scattering
of light. Depending upon whether or not the energy of photon (or the color of the light) changes during the process of
scattering, it is classified as either elastic or inelastic scattering. In an elastic scattering, where the photon energy is
conserved, the photon moves away with the same energy (or color) that it came with, even if it had an interaction with the material.
Such an elastic scattering of light is known as the Rayleigh scattering. On the other hand, the incoming
photon may lose some part of its energy to the material (or gain some energy from the material) during the process of collision,
and moves away with a different energy (or color). Such an inelastic scattering is known as the Raman scattering.
Even though there are very few photons that go through Raman scattering (approximately 1 in 10 million photons), this process is
extremely interesting to study, because this process involves energy transfer between the photon and the material, hence the change of
color of the photon carries signatures of various intrinsic properties of the material. The material gains (loses) energy from (to)
the photon in the form of its vibrational energy. Therefore, the change of energy (or color) of the light during the Raman
scattering process is the same as the vibrational energies of the material. In fact, the vibrational energies of the materials
that we deal with in our day-to-day life are about 100 times smaller than the energy carried by the visible light. Thus the
photon must lose only a small part of its energy to excite the material from its one vibrational energy level to another,
which can be possible only through an intermediate process. The intermediate process involves virtual electronic energy levels,
which can be understood as any random linear combination of real electronic energy levels. As it can be understood from the
figure, the interaction between photon and sample takes the sample to a virtual electronic energy level that coincides with
the energy of the incoming photon. This is an intermediate situation, from where the sample comes down to an energy level
that is an excited level of the vibrational energy. The photon then moves away with the remaining energy. This all happens
in one single process of the collision (scattering), and hence there is no absorption and reemission of the photon. Rather,
it is the same photon that moves away with a changed energy. The most common process during light scattering, the Rayleigh
scattering, is shown in Fig. (a), where the photon scatters away with the same energy that it came with. Inelastic scattering
can involved either loss or gain in the photon energy, which are shown in Figs. (b) and (c), which are also known as the Stokes
and Anti-Stokes scattering, respectively. The two inelastic scatterings are known as the Raman scattering.
In some particular case, the virtual electronic energy levels may match the real electronic levels. In those case, the
scattering process becomes much stronger, and is referred as the resonant Raman scattering.
Since Raman scattering involves both, the vibration and the electronic energy levels of a sample, it is very rich in
information related to the intrinsic properties of the sample.