Raman spectroscopy

 

Ir. Sugeng Endarsiwi (Printing Ink Manufacturer)

What is Raman spectroscopy?

Raman spectroscopy is a chemical analysis technique which involves illuminating a substance with a laser and analyzing the light that is scattered off the surface of the substance. The scattered light can provide a lot of information about the substance and its structure, and can be used to identify, characterize, and quantify many chemical components. 

Light scattering explained

When light is scattered off a sample there are two possible outcomes:

(1) Elastic scattering, also known as Rayleigh scattering, occurs when the scattered light has the same energy as the light that initially struck the sample. This means that the elastically scattered light will be the same frequency, wavelength, and color as the original beam of light.

(2) Inelastic scattering, or Raman scattering, occurs when the scattered light has a different energy than the light that initially struck the sample. This means the inelastically scattered light will have a different frequency, wavelength, and color than the original beam of light. 

As the name implies, this is the type of scattering that gives Raman spectroscopy its name. So why is this kind of scattered light so useful for spectroscopy, and what causes it to have a different energy after interacting with a sample?

Applying the Raman effect

Since the frequencies of light absorbed when a molecule is illuminated are unique to the molecule and type of bonds, detecting these frequencies of light will allow us to figure out which molecules are present in the sample.

This is the aim of Raman spectroscopy.  

So how can we detect which frequencies of light were absorbed by molecules in the sample? Well, when those molecules absorb some of the light (inelastic scattering), the frequency of the light changes. So to detect the Raman effect, we can simply determine the frequency shift between the original beam of light and the Raman scattered light. The frequency shift is called the Raman shift.  

By using a monochromatic laser for the experiment we can easily determine the frequency of the original light beam, since lasers emit light that is all the same wavelength and frequency. The typical choice is a green laser (532 nm). To make it clear: When the sample is illuminated with the laser, some of the laser light will be absorbed by the sample to excite molecular vibrations, causing Raman scattering. The Raman scattered light is then collected at a detector so we can determine its frequency.  That will give us all the information we need to determine the Raman shift.

Source: https://www.bruker.com/en/products-and-solutions/raman-spectroscopy/raman-basics/what-is-raman-spectroscopy.html