A Basic Overview of Raman Spectroscopy

Raman spectroscopy is a non-destructive laboratory technique, which provides information about chemical structure, phase, polymorphy, and crystallinity through interactions with molecules.

This article brings light to the outline of Raman spectroscopy, which is requisite to deal with the principle of Raman imaging, Raman instrument, and application of Raman spectra.

Spectroscopy

In different ways, light interacts with matter. It transmits through some materials, at the same time, it reflects or scatters off from some other materials. In addition, luminescence and absorption also happen. This study of this light is spectroscopy, which is a part of the visible spectrum that enters our eyes and determines which colors we perceive.

A substance may appear blue if it absorbs the red part of the spectrum of light falling upon it and reflects or scatters the blue parts into our eyes.

Raman Scattering Process

If the shining blue light is required, from one part of the spectrum to the material, the blue light is reflected from it or no light is coming out if it is completely absorbed (black material). But, when a Raman spectrometer is used, a very tiny fraction of the scattered light has a distinct color. It has changed frequency since during the time of scattering process, the energy changed by interaction with molecular vibrations. This is the Raman scattering process. It is named after the popular Indian physicist, C. V. Raman. For his distinguished discovery, he got the Nobel prize in 1930.  

Through the study of the vibration of atoms, the chemical composition of compounds and other useful information can be discovered.

As only 1 part in 10 million of the scattered light has a color shift, the Raman effect is weak. Since this is too weak, it is impossible to see with a naked eye. Hence, the light is analyzed using a highly sensitive spectrometer.

Basic components of Raman Spectrometer

Raman spectrometer system consists of:

• Laser - one or more colored light source

• Lenses - focussing the light onto the sample and collection of scattered light

• Filters - Purification of reflected and scattered light for the collection of Raman light only

• Diffraction grating or prism - Light splitting into constituent colors

• Sensitive detector - Detection of weak light

• Computer - to control the whole system, display the spectrum and provide information for analysis.

Raman Scattering

When light interacts with molecular vibrations, Raman scattering takes place. This is somewhat similar to the infrared absorption spectroscopy, but rules are different. A change in molecular polarizability is a requisite during the vibration of Raman effect to take place. 

Owing to the different selection rules, some vibrations in the Raman spectrum are not visible in the infrared spectrum and vice-versa. Example is Raman spectroscopy is one of the best tools to study the carbon atoms, which make up the structure of diamond. However, infrared spectroscopy is not a good technique to study the same.

Scattered light

First step is production of Raman spectrum is illumination of samples with monochromatic light sources like lasers. Most of the material that scatters off the matter is unchanged in energy or Rayleigh scattered. A tiny fraction, 1 in 10 million has gained or lost energy, which is Raman scattered. The Raman shift materializes as photons, that is to say particles of light and exchanges part of their energy with molecular vibrations in the material.

When the energy is lost, the Raman scattering is entitled as ‘Stokes’. At the same time, when the energy is gained, the Raman scattering is classified as “anti-Stokes''. Though anti-Stokes Raman light represents equivalent vibrational information of molecules, it is rarely used as it is less intense than the Stokes.

Vibrating atoms

Depending on the frequency of the molecule, the change in energy occurs. If there is high frequency and change is fast, light atoms are held together with strong bonds and the energy change is significant. If there is low frequency and change is slow, the atoms are held together with weak bonds and the energy change is negligible.

Raman Spectra

Results obtained from Raman spectrometers are graphically depicted as Raman spectra. Rotational Raman spectra, vibrational Raman spectra and Electronic Raman spectra can be obtained. The intensity of the scattered light on the Y-axis is plotted against the energy or frequency of light on the X-axis. The frequency is measured in wavenumber, which is the number of waves per cm, cm^-1.X-axis frequencies relative to that of laser are plotted as it is the shift in energy of the light, which is the desired one.

From the Raman spectrum, a plethora of information can be obtained.

With the Rama shifts and relative intensities of Raman bands of the material, material can be identified.

Individual band changes as a band may narrow, broader or shift, or vary in intensity. These changes disclose information about the stresses in the sample, changes in crystallinity and the amount of material.

Variation of spectra with position in the sample reveals change in the homogeneity or uniformity of the material.

Further, the spectrum can be analyzed at several arbitrary points and systemically measured at an array of points, which validates the production of images of compression, stress and crystallinity.

Raman bands

Crystals having the same configuration and a regular array of identical atoms will display only one dominant Raman band. The same configuration is like the carbon atoms in a diamond. There is only one molecular arrangement of the crystal.  

Polystyrene is a less symmetric molecule. In addition to carbon atoms, it has hydrogen atoms too. Connecting the atoms is done by different types of bonds. Hence, the Raman spectrum of polystyrene is complex.

Conclusion

A slight introduction to the spectrum has been given. Then the origin of Raman spectroscopy has been discussed. The list of components and their purpose in the Raman spectrometer has been given. Raman scattering has been briefed and scattered light has been detailed. Subsequently, Raman spectra and their uses have been explained. Finally, differences in Raman bands in two different materials have been illustrated.