Raman Effect

Different molecules have different rotational and vibrational properties. The Raman Effect deals with the scattering of molecules, producing photons of different frequencies depending on the vibrational and rotational properties. Chemists and physicists use this property to study different materials present around us.

Scientists in earlier days used to study materials with a mercury lamp that used to produce spectra on the photographic plates. In modern days, photographic plates are replaced by lasers. Sir CV Raman got the Nobel Prize in Physics for Raman scattering principle in 1930, with his student KS Krishnan.

Raman Scattering Effect

Scattering is the principle by which photons of molecules get excited to higher energy levels. Sir CV Raman observed this scattering principle. Hence, the effect was named after him. The photons scatter inelastically. It means the kinetic energy of an incident particle is either increased or lost. In addition, the kinetic energy also comprises Stokes and anti-Stokes portions.

The inelastic scattering of photons is very much like an inelastic collision. According to the inelastic collision in photons, the total microscopic kinetic energy is not conserved. It means, during the collision of photons, the kinetic energy transfers from one photon to another. However, during this process, the scattering will remain inelastic as it happens in Compton scattering.

Raman scattering depends upon the polarizing properties of molecules, which is similar to Rayleigh's scattering principle. The intensity of Rayleigh scattering is 10-3 to 10-4 compared to the source's intensity which excites the photons. The state and energy of photons within a molecule remains unchanged. The frequency of photons in the monochromatic light changes in the Raman Effect. It happens when the photons interact with vibrational states or modes of a molecule.

Laser is one of the sources considered an intense source of light in the Raman scattering principle. It gives rise to scattered light containing more than one sideband differed by vibrational and rotational differences in energies. Several frequencies are included in the sidebands produced, which contain more information about the scattering medium. These sidebands are used in remote sensing these days to capture a large amount of data.

Degrees of freedom

Degrees of freedom in a system is defined as the number of parameters needed to determine the physical system's configuration. The degrees of freedom are represented by:
DF = n – 1
Where n = number of samples

In the Raman Effect, the degrees of freedom is given by 3N, where N is the number of atoms in any chemical compound. The number 3 is used as any particle can rotate along three axes, x, y and z, i.e. they possess rotational, vibrational and translational motion.

Raman spectroscopy

In 1928, two years before discovering scattering, Sir CV Raman discovered spectroscopy, named after him. He used spectroscopy to study vibrational, rotational and low-frequency modes of the molecules. Spectroscopy is widely used these days by chemists and forensics to solve crime cases.

Principle of Raman spectroscopy

The concept of monochromatic radiation passing through the sample, such that the source radiation gets reflected, absorbed or scattered. This principle is used in Raman spectroscopy. The scattered photons of the molecules have varying frequencies from that of the incident light source. This change in frequency results in the change in wavelengths, which is later studied under IR spectra.

Raman shift denotes the difference between the incident photon and the scattered photon. If the energy of the scattered photons is less than the energy of an incident photon, it is known as Stokes scattering. On the other hand, if the energy of the scattered photons is more than the incident photon's energy, then the scattering is known as anti-Stokes scattering.

Types of Raman spectroscopy

1. Surface-enhanced Raman spectroscopy (SERS)
2. Resonance Raman spectroscopy (RRS)
3. Micro Raman spectroscopy
4. Non-linear Raman spectroscopy

Raman spectrometer

An instrument consisting of more than one single coloured light source, lenses and filters, so that one can focus and differentiate between the incident, reflected and scattered light is known as a Raman spectrometer. Most commonly, a prism is used for splitting the light into its seven constituents. The spectrometer has a detector to detect weak light. A monitor is attached to the spectrometer to see and analyze the scattering information.

Applications of Raman Effect

1. Raman Effect is most widely used in the telecom industry. A lower-frequency particle is amplified into a higher-frequency particle for transmitting information to far off places in telecommunication.
2. Raman Effect is used in optics.
3. It is widely used in nanotechnology, studying DNA and proteins and understanding the structure of atoms and their bonds with other atoms or molecules.
4. Raman Effect is used in remote sensing, planetary exploration, and finding minerals in different solar system planets.