Overview of Raman Spectroscopy: Fundamental to Applications
147
Note: The above historical overview is based on the article cited [2] by R. S. Krishnan
and R. K. Shankar.
After the discovery of the Raman effect, physicists and chemists started working
on a wide range of problems and contributed more knowledge in both chemistry and
physics field. Additionally, the development of laser boosts the development of the
Raman spectroscopy for the characterization of materials in its different forms. The
present chapter briefly explains the theory of Raman spectroscopy, instrumentation
and the applications in the diverse fields of research.
2 Theory of Raman Spectroscopy
Raman spectroscopy is the inelastic scattering of light by the object. The molecule
gets excited while interacting with the incident light which results in the distortion of
the electron cloud. If the distorted electron cloud acquired its original state by emitting
the photons having the same frequency of the incident radiation, the phenomenon
is called the elastic scattering, which is also admitted as the Rayleigh scattering
[3]. However, when the incident light causes nuclear motion, during the scattering
process, the energy transfer takes place between the molecule and the photon (either
molecule to the photon or photon to the molecule). This process is recognized as
the inelastic scattering of the radiation or the Raman scattering. This process is
very weak, as the one photon is scattered from every 10
6 –10
8 incident photons in
inelastic scattering [3]. Therefore, it is quite difficult to detect the scattered photons,
which further leads to the slow development of Raman spectroscopy. However, the
discovery of laser in 1969, raises the growth of the Raman effect and now a day it is
widely used in almost all fields of research in science and technology.
The theory of Raman scattering is explained through both the classical and
quantum mechanical approach. The classical theory treated the electromagnetic
waves and the scattering materials classically. However, the classical theory does
not rule out some of the aspects of frequency dependence and the intensity of the
Raman scattering. While the quantum mechanical approach to the Raman scattering
provides the insights of intensity and the selection rules as discussed in the following
sections.
2.1 Classical Theory
The classical theory is based on the induced electric dipole, which relies on the
vibrational frequency and the electric field of the incident radiation. When incident
radiation is passed through the material, the scattering of light occurs. Further, the
interaction between the incident light of the electric field and the molecules generates
electric dipoles, having permanent dipole moments. The induced electric dipole ( p)
in the presence of incident radiation is given by
147
Note: The above historical overview is based on the article cited [2] by R. S. Krishnan
and R. K. Shankar.
After the discovery of the Raman effect, physicists and chemists started working
on a wide range of problems and contributed more knowledge in both chemistry and
physics field. Additionally, the development of laser boosts the development of the
Raman spectroscopy for the characterization of materials in its different forms. The
present chapter briefly explains the theory of Raman spectroscopy, instrumentation
and the applications in the diverse fields of research.
2 Theory of Raman Spectroscopy
Raman spectroscopy is the inelastic scattering of light by the object. The molecule
gets excited while interacting with the incident light which results in the distortion of
the electron cloud. If the distorted electron cloud acquired its original state by emitting
the photons having the same frequency of the incident radiation, the phenomenon
is called the elastic scattering, which is also admitted as the Rayleigh scattering
[3]. However, when the incident light causes nuclear motion, during the scattering
process, the energy transfer takes place between the molecule and the photon (either
molecule to the photon or photon to the molecule). This process is recognized as
the inelastic scattering of the radiation or the Raman scattering. This process is
very weak, as the one photon is scattered from every 10
6 –10
8 incident photons in
inelastic scattering [3]. Therefore, it is quite difficult to detect the scattered photons,
which further leads to the slow development of Raman spectroscopy. However, the
discovery of laser in 1969, raises the growth of the Raman effect and now a day it is
widely used in almost all fields of research in science and technology.
The theory of Raman scattering is explained through both the classical and
quantum mechanical approach. The classical theory treated the electromagnetic
waves and the scattering materials classically. However, the classical theory does
not rule out some of the aspects of frequency dependence and the intensity of the
Raman scattering. While the quantum mechanical approach to the Raman scattering
provides the insights of intensity and the selection rules as discussed in the following
sections.
2.1 Classical Theory
The classical theory is based on the induced electric dipole, which relies on the
vibrational frequency and the electric field of the incident radiation. When incident
radiation is passed through the material, the scattering of light occurs. Further, the
interaction between the incident light of the electric field and the molecules generates
electric dipoles, having permanent dipole moments. The induced electric dipole ( p)
in the presence of incident radiation is given by