Unlike other spectroscopic techniques which are based on the absorption or emission of light energy by a molecule, Raman Spectroscopy is based on the phenomena of scattering of light. It uses the inelastic scattering of light, that is there is a change in the wavelength of the light interacting with the sample molecules.
Homonuclear diatomic molecules (O₂, N₂, H₂, etc.) that cannot show Infrared spectra, show Raman spectra since a change in polarisability of the molecules accompanies their vibration. As a consequence of the change in polarisability, there occurs a change in the induced dipole moment at the vibrational frequency.
According to the mutual exclusion rule, centrosymmetric molecules, the vibrations that are active in Infrared spectroscopy, are inactive in Raman spectroscopy. While the vibrations which are inactive in Infrared spectroscopy are active in Raman spectroscopy.
Raman spectroscopy is named after the Indian physicist C.V. Raman who, together with his research partner K.S. Krishnan observed Raman scattering in 1928.
Principle of Raman Spectroscopy
When the incident light from a laser source is illuminated on a substance, the molecules of the substance scatter the light in a direction at right angles to the incident light in two ways. Most of the scattered light is at the same wavelength as the laser source and does not provide useful information– this is called Rayleigh Scatter.
However, a small amount of light is scattered at different wavelengths, which depend on the chemical structure of the analyte – this is called Raman Scatter.
Rayleigh scatter ⟶ 𝜆(scatter) = 𝜆(laser)
Raman scatter ⟶ 𝜆(scatter) > 𝜆(laser)
Raman scattering can be further divided into Stocks Raman scatter and Anti-Stocks Raman scatter.
If the molecule gains energy from the photon during the scattering (excited to a higher vibrational level) then the scattered photon loses energy and its wavelength increases which is called Stokes Raman scattering. Inversely, if the molecule loses energy by relaxing to a lower vibrational level the scattered photon gains the corresponding energy and its wavelength decreases; which is called Anti-Stokes Raman scattering.
Rayleigh scatter is the most common phenomenon while anti-stokes scattering is the least common. It is because in Rayleigh scatter there is no change in the vibrational energy, on the other hand in anti-stokes scatter the molecule must be in a vibrationally excited state before the photon is incident upon it.
Another important phenomenon observed in this is the Raman shift. When photons interact with a molecule, the molecule may be advanced to a higher energy, virtual state. From this higher energy state, there may be a few different outcomes.
One such outcome would be that the molecule relaxes to a vibrational energy level that is different from that of its beginning state producing a photon of different energy. The difference between the energy of the incident photon and the energy of the scattered photon is called the Raman shift.
Instrumentation of Raman Spectroscopy
Raman spectroscopy is carried out by a very simple instrument, and it has various components that include:
I. Laser Sources
It is usually a solid-state laser with popular wavelengths of 532 nm, 785 nm, 830 nm, and 1064 nm. It produces high-intensity laser beams that are not absorbed by the sample molecules.
II. Sample Illuminating System
The laser beam from the source is illuminated on the sample that scatters the incident radiations. These scattered radiations are collected with a group of lenses. The sample illuminating system determines the phase of the sample.
III. Wavelength Selector
This system helps in converting a polychromatic light into a monochromatic light of a specific wavelength. It is usually a spectrometer that is placed 90° away from the incident illumination and may include a series of filters or a monochromator.
Raman scattered radiation is dispеrsеd using gratings with 1200 grooves/mm and finally passes through the monochrome.
The detector captures the light, resulting in the Raman spectrum. Since Raman scattering yields a weak signal, it is most important that high-quality, optically well-matched components are used in the Raman spectrometer.
The most simple detector used in the instrument is a photomultiplier tube. The photomultiplier tube is placed in a thermoelectric cooler (-30ºC), markedly lowering the dark current and reducing noise, thus providing a high sensitivity and favorable signal-to-noise ratio.
V. Amplifier and Recorder
As the signals are weak, they are amplified and counted with two photon-counting systems, one of which is for the detection of the ordinary Raman spectrum, while the other is for the calculation of the depolarization ratio. These signals are displayed graphically as a function of wavelength or frequency.
Samples Analyzed Through Raman Spectroscopy
All samples that contain true molecular bonding can be easily analyzed by this method. That is all the solids, powders, slurries, liquids, gels and gases can be analyzed using Raman spectroscopy.
Although gases can be analyzed using Raman spectroscopy, the concentration of molecules in a gas is typically very low, so the measurement is often more challenging.
Usually, specialized equipment such as higher powered lasers and long path length sample cells are necessary. In some cases where gas pressures are high, standard Raman instrumentation can easily be used.
Why Raman Spectroscopy??
The Raman spectrum is a distinct chemical fingerprint for a particular molecule or material and can be used to very quickly identify the material, or distinguish it from others.
It elucidates the information about the sample’s chemical structure, identity, phase, polymorphism, intrinsic stress/strain, contamination, and impurity.
Raman spectroscopy is a qualitative as well as quantitative analytical technique. It is a spectroscopic method that is based on the scattering of light rather than the emission or absorption of the light by the sample molecules.
It is one of the most modern spectroscopic techniques utilized in various scientific disciplines. It studies the vibrational energy of the atoms in a sample, similar to Infrared spectroscopy but it is preferred over the latter due to a few advantages.