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Specifically, the Raman effect is a light scattering technique based on the principle that molecules scatter incident light from a highly intense light source, typically a laser source. The scattered, incidental light is usually the same wavelength as the laser light and does not provide essential data about the sample. That’s because when light hits something, most of the photons that are scattered are the same energy as the light it was hit with. This scattered light is also referred to as Rayleigh Scatter.
However, a minimal amount of light is scattered at a different wavelength than the Rayleigh Scatter and is indicative of an analyte’s chemical structure. This shorter wavelength scattered light is called Raman Scatter, or Raman effect. It is named after Sir C.V. Raman.
In other words, Raman spectroscopy is a type of molecular spectroscopy that relies on inelastic scattering (Raman Scatter) of light to detect vibrational, rotational, and other low-frequency modes in a sample.
It is used in chemistry to identify molecules by providing a structural fingerprint, represented by a Raman spectrum. This spectrum is characterized by several peaks that describe the intensity and wavelength position of the scattered light from a sample and are unique to specific molecules and materials. Each peak relates to a particular molecular bond vibration, including both individual bonds and groups of bonds.
Raman spectral libraries are often used to identify a sample based on its Raman spectrum. The libraries contain thousands of samples’ Raman spectra, collected from various positions of each sample.
Because Raman looks at the scattering of light rather than its absorption, the sample preparation is much less complex, and there are no aqueous absorption bands to throw off the data. When a monochromatic laser interacts with a sample, the light will scatter. Furthermore, these devices offer both low/medium resolution and high-resolution capabilities.
These devices’ instrumentation varies depending on the experimental capabilities you wish to have. We’ll review the basics. Furthermore, Raman spectrometers rely on different techniques to observe change, which will also be explained in further detail below. We’ll cover some of the most common ones, as several different types of Raman spectroscopy are being used today.
Raman spectrometers are made of three main components:
The laser’s excitation wavelength can vary depending on the sample being analyzed using Raman spectroscopy. The various wavelengths include some typical examples: ultraviolet, visible, and near-infrared (NIR). The one you choose will depend on your goals and the experimental capabilities you wish to have.
In many spectrometers today, the sampling interface is a type of fiber-optic probe. It can be augmented to fit a range of optical microscopes, liquid flow cells, gas flow cells, and various sampling chambers.
The spectrometer provides high resolution and low power consumption without much noise and includes the detector. The detector type depends on the laser source being used. When visible light is used, a standard charge-coupled device (CCD) detector is utilized; however, various CCDs exist that are optimized for specific wavelengths. In the case of UV excitation, a particular CCD detector is used, along with objective lenses and diffraction gratings.
A highly accurate and powerful technique, surface-enhanced Raman spectroscopy (SERS) improves normal Raman scattering by huge orders of magnitude and can be used to detect single molecules.
This technique applies Raman intensity and improves Raman scattering through an electromagnetic amplification mechanism. The enhancement occurs on the surface of a rough metal substance or via nanostructures consisting of magnetic-plasmonic silica nanotubes.
Due to its ability to identify chemical species and analyze the makeup of a mixture on the nanoscale, this technique is used extensively in chemistry, pharmaceuticals, and materials science.
Coherent anti-Stokes Raman Spectroscopy (CARS), also known as coherent anti-stokes Raman scattering, is similar to conventional Raman spectroscopy. It is also used to measure the vibrational signatures of molecules.
However, it differs from the other technique because it employs multiple photons to address the vibrations. Doing so produces a coherent signal or identical wave sources. This makes CARS much stronger than processes like spontaneous Raman emission.
Fourier Transform (FT) Raman spectroscopy relies on a specific type of configuration, which is designed to collect wavelength-stable and fluorescence-free measurements. A conventional FT Raman spectrometer comprises an excitation laser source, a sampling interface, and an interferometer.
The use of an interferometer makes FT Raman distinct from dispersive techniques, such as Dispersive Raman, which uses a diffraction grating spectrometer to disperse the light scattered from a sample. While the diffraction grating will detect the scatter via a CCD detector, producing a Raman spectrum directly, the FT Raman spectrometer’s interferometer will introduce a path between the light source and the signal beams, creating an interference pattern. This pattern is used to reconstruct the Raman spectrum.
By using a specific wavelength to cause scattering, resonance Raman spectroscopy can increase the intensity of Raman scattering. This is done by choosing a wavelength that either overlaps or is extremely close to the electronic transition of the sample that is being observed.
Due to resonance Raman spectroscopy’s increased intensity, it can detect samples with extremely low concentrations in a substance. One major disadvantage of this technique is that the fluorescence of an object may throw off the data collected and should be accounted for. This makes it an instrumental technique in analyzing environmental pollutants that have concentrations in the parts per billion range.
By shining light through a sample in the direction of the excitation laser, transmission Raman spectroscopy allows for bulk analysis of powders, tablets, and opaque substances.
By shooting the light through the object and analyzing the light that comes out the other side, it allows for analysis of the entire volume of the material.
Their ability to perform fast, quantitative analysis of substances makes them useful in pharmaceutical and medical analysis, as well as material sciences.
Also known as spontaneous vibrational Raman optical activity scattering, this vibrational spectroscopy technique looks at the difference in intensity of Raman scattered from the right and left circularly polarized light.
Similar to vibrational circular dichroism, Raman optical activity directly looks at chirality, or molecular vibrations. Due to its ability to observe chirality, this spectroscopic technique is very useful in chemistry and biology.
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