What Is The Basic Principle of Raman Spectroscopy?

What is Raman Spectroscopy?

Raman spectroscopy is one of the popular laboratory techniques used for the analysis of molecular structure. It is considered complementary to infrared spectroscopy. Indian Physicist Chandrasekhara Venkata Raman identified the Raman effect in 1928, on the basis of which, Raman spectroscopy is operated. Raman imaging and Raman analysis are also the terms used for Raman spectroscopy. This write-up elaborates on the principle and basis on which Raman spectroscopy is operated..

Principle Basics

• The Raman effect is based on the scattering of light, which consists of elastic scattering at the same wavelength as the incident light and inelastic light at different wavelengths due to molecular vibrations. The elastic scattering is Rayleigh scattering and inelastic scattering is Raman scattering.
• The Raman shift is the energy difference between incident light and scattered light. The Raman spectrum is notated as the intensity of scattered light in the vertical axis and the wavenumber of the Raman shift in the horizontal axis.
• Raman shift is the shift in wavelength of the inelastically scattered radiation, which provides structural and chemical information. Raman shifted photons can be of lower or higher energy, depending on the vibrational state of the molecule under observation. Rayleigh scattering is elastically scattered radiation, which results in a change of phase, but the frequency is not shifted.
• The Raman shift is involved in two different energy bands. The shift in wavelengths higher than that of the incident light is termed as Stokes scattering. Similarly, the shift in wavelengths lower than that of the incident light is named anti-Stokes scattering. Stokes scattering can be observed in the lower wavenumber or longer wavelength region. Anti-stokes scattering is observed in higher wavenumber or shorter wavelength regions. Generally, Stokes scattering is used for analysis, however, sometimes, Anti-stokes scattering is also used.
• Often Stokes line and anti-Stokes lines are displaced equally from the Rayleigh line. It occurs as in either case, one vibrational quantum of energy is lost or gained. Anti-Stokes is less intense than the Stokes line. This takes place because only molecules that are vibrationally excited prior to irradiation give rise to the anti-Stokes line. So, in Raman spectroscopy, only the more intense Stokes line is usually measured as Raman scattering is a relatively weak process. The number of photons scattered in Raman scattering is quite lesser in number.
• Raman scattering is a million times less intense than Rayleigh scattering. Hence, to get Raman spectra, it is essential to prevent Rayleigh scattering from overpowering the weaker Raman scattering.

• Stokes radiation takes place at lower energy than the Rayleigh radiation, whereas anti-Stokes radiation occurs at greater energy than the Rayleigh radiation. The decrease or increase in energy is related to vibrational energy levels in the ground electronic state of the molecule. Hence, the wavenumber of Stokes and anti-Stokes lines indicate the direct measure of vibrational energies of the molecule. 
• Anti-Stokes bands are measured in fluorescing samples as fluorescence causes interference with Stokes bands. The magnitude of Raman shifts is not dependent on the wavelength of incident radiation. However, the Raman scattering is dependent on the wavelength of incident radiation. 
• In the Raman effect, the frequency of a small fraction of scattered radiation is different from that of monochromatic incident radiation, which is based on the inelastic scattering of incident radiation through its interaction with vibrating molecules. Because of inelastic collision between incident monochromatic radiation and molecules of the sample, the spectrum arises. Usually, the scattered radiation is measured at right angles to incident radiation.
• In a Raman spectrophotometer, glass is used for optical components, like sample cells, mirrors, and lenses. A change in polarizability at the time of molecular vibration is an essential requisite to obtain a Raman spectrum of the sample. Water is mainly used for dissolving samples, as Raman scattering because of water is low.. Water vibrations show low-scattering cross sections allowing easy recording of Raman spectra in aqueous solutions, which makes Raman spectroscopy a perfect candidate for label free in vivo investigations on a molecular level. Only one photon out of 108 photons is scattered inelastically, so the sensitivity is lower and is characterized by small Raman cross-sections.
• The Raman effect depends on the molecular deformations in electric field E, which is influenced by molecular polarizability (α). The laser beam can be regarded as an oscillating electromagnetic wave with an electrical vector E. It induces an electric dipole moment P=αE and deforms the molecules after interacting with the sample molecules.  An example is the change of polarizability during CO2 vibrations.
• Because of the periodic deformation, molecules vibrate at a characteristic frequency. So, the monochromatic laser beam having a frequency excites molecules and converts them into oscillating dipoles. These dipoles produce light of dissimilar frequencies. 
• The intensities in the bands of Raman spectra are dependent on the nature of vibration, instrumentation and sampling factors. 
• The vibrational excitation in Raman spectroscopy takes place via a two-photon scattering process. As molecular vibration is distinct for each molecule, vibrational spectra can be interpreted as the type of molecular footprint of an examined molecule. The molecule can be inorganic, organic, biological cell, biological tissue.
• The photons of light are absorbed by the sample and then reemitted. The frequency of the reemitted photons goes up or down with respect to primary monochromatic frequency. The shift contains some valuable information about the rotational, vibrational, and low-frequency transitions of the molecules.

Conclusion

In this article, a short introduction has been given for Raman spectroscopy. Basic principles are discussed. Rayleigh scattering and Raman scattering have been explained. The concept of Stokes scattering and anti-Stokes scattering have been provided. The Raman effect has been detailed.