Raman Spectroscopy

What is it?

Raman spectroscopy is a qualitative and quantitative technique used to identify chemicals by measuring spectral data and comparing the results to digital databases. The Raman spectrometer is comprised of four major components: a laser which serves as the excitation source, an illumination system, a wavelength filter, and a detector. Though there exists a variety of detectors as well as instrument add-ons this captures the basic functions of the instrument. The analysis process starts with the illumination of the sample by the laser beam. The resulting scattered light is then channeled through a lens into an interference filter and the wavelengths are measured by the detector. A computer then produces a spectral data report which provides the information necessary to identify the compound by the calculated energy changes.

What is it used for?

Vibrations caused by polarization energy are unique to their chemical bonds and the molecule’s symmetry. Therefore measuring this energy provides the necessary information to characterize and identify the unknown molecular make up of a sample. This process is utilized to do everything from identify a material, measure its temperature or find crystallographic orientation. This type of analysis is used in a variety of fields. Historians and archeologist use Raman spectroscopy to investigate historical documents and use the chemical findings to make inferences about the social and economic conditions at the time of the document’s creation. As Raman spectroscopy is a non-invasive technique it provides beneficial knowledge without damaging the materials in question. This testing method can also be used to do everything from detect explosives from a safe distance using laser beams, to discover counterfeit drugs without ever opening the packaging. One of the more common uses for this technique is remote monitoring of pollutants. For instance, directing the laser beam on exhaust fumes and measuring the scattering enables monitoring of molecular levels of chemical pollutants. Due to this capability of remote sensing, Raman spectroscopy may become a major analytical tool for planetary exploration analysis.

How does it work?

Inelastic scattering of monochromatic light provides the basis for Raman spectroscopy. When the laser emits photons the light is absorbed by the sample and then reemitted. Depending on the sample’s make up the emitted frequency of light will change from the original monochromatic frequency. This shift is what is called the Raman Effect. It is this shift in frequency which provides the necessary information about molecules’ vibrational, rotational, and other transitional properties. This process can evaluate solid, liquid, and gaseous samples.

The image depicts the process of Raman scattering. The light emitted from the laser beam is scattered at the sample’s surface. The majority of this light is scattered at the same wavelength as the incident light (called Rayleigh scattering), but a small fraction (0.001%) is shifted and scattered at a changed wavelength. These shifted waves are referred to as Raman scattering and are due to the light’s interaction with the sample’s molecules. A lens collects the light, and a filter separates the Raman scatterings from the incident light. These shifted wavelengths then traverse the monochromator and detection system which measures their frequency. The specific frequencies of the shifted scattered light reveal the molecular structure of the sample material.

The aforementioned scattering is due to what is called the “Raman effect” which occurs when light interacts with a molecule’s electron cloud and causes a nuclear displacement. The emitted frequency is determined by the rotational and vibrational state of the molecule. There are three possible frequency type that such an oscillating dipole can emit.

1. The most common scattering is elastic scattering, called Rayleigh scattering. This happens when a molecule with no Raman-active modes absorbs a photon, exciting the molecule and then returns back to the original state, thus emitting light with the same frequency as the light source.

2. If the molecule is Raman-active and in basic vibrational state, the molecule absorbs part of the photon’s energy, emitting a lessened frequency. This is called a Stokes frequency.

3. If the molecule is Raman-active and in an excited vibrational state, the molecule will release added energy to that of the photon, emitting a higher frequency as the molecule returns to the basic vibrational state. This is called an Anti-Stokes frequency.

These frequency changes are all dependent upon the vibrational and rotational states of the sample. The polarization change is directly related to the deformation of the electron cloud and produces Stokes and Anti-Stokes frequencies accordingly. These frequencies are plotted based on their intensity in respect to the Rayleigh frequency. Plotting the emitted light in this way positions the bands at frequencies corresponding to the vibrations of different functional groups. The spectra created by this method can be interpreted similarly to that of infrared absorption spectrum.

 

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