What Are The Types Of Raman Spectroscopy?

Raman Spectroscopy

Raman spectroscopy is a laboratory analytical technique in which scattered light is used to measure the vibrational energy modes of a sample. This technique is named after the renowned Indian physicist C. V. Raman, who was the first person to observe Raman scattering or Raman shift in 1928 along with his research partner K. S. Krishnan.

This article takes you to the different variants of Raman imaging. Some of them are enhanced techniques. Their working principle and usage have been discussed.

Types of Raman Spectroscopy

More than 25 various types of Raman spectroscopy imaging techniques have been discovered. Among them, some have emerged popular. Let us see some major techniques.

Surface Enhanced Raman Spectroscopy (SERS)

• SERS is developed to overcome some of the disadvantages exhibited by conventional Raman spectroscopy. Conventional Raman spectroscopy is a relatively underdeveloped technique, in which degradation of samples takes place. 
• SERS leads to an enormous increase in intensities of Raman signals. Besides, it can measure the adsorption of molecules on nanostructures.
• Electromagnetic enhancement takes place. Incident light creates localized surface plasmons and the plasmons oscillate, leading to an increase of the scattering with perpendicular oscillations.
• Also incident light enhances chemisorption of the surface. Charge transfer occurs, which leads to enhancement of scattering.

Tip Enhanced Raman Spectroscopy (TERS)

• Raman spectroscopy is brought into nanoscale resolution imaging by TERS. It is a super-resolution chemical laboratory technique. It is usually performed with AFM and Raman system, where Scanning Probe Microscope (SPM) is integrated with a confocal Raman spectroscopy via an optomechanical coupling. The optical coupling brings the excitation laser to the probe or functionalized tip and the spectrometer analyzes the scattered light with a nanometer scale.
• There are two different configurations for TERS. One is transmission and another is reflection. 
• Transmission mode uses higher numerical aperture (NA) objectives, providing high power density at the focus point, enabling the collection of high signal levels. The downside is the technique is applicable for transparent samples only.
• The reflection mode can be applied for both opaque and transparent samples. But it is limited to lower NA objectives.

Surface Plasmon Polariton Enhanced Raman Scattering (SPPERS)

Surface plasmon polaritons (SPP) are electromagnetic modes, which are propagating at the interface between a negative and positive permittivity medium because of resonant oscillations of free carriers. They have the characteristic to confine photon energy into subwavelength volumes at the conductive or dielectric interface where the electromagnetic evanescent field is strongly increased. This property has increased the detection and imaging capabilities. So, SPPs are utilized to perform remotely excited SERS, which results in a technique where excitation and SERS signal collection are displaced spatially. This reduces the origination of fluorescence background signals. Thus, the term Surface Plasmon Polariton Enhanced Raman Scattering (SPPERS) has been derived.

Surface Enhanced Resonance Raman Spectroscopy (SERRS)

SERRS is a selective and sensitive method used for the characterization of sites in biomolecules, which possess an electronic transition at an energy close to the laser frequency used. Here, the sensitivity of resonance with that of the SERS technique is combined, so very low concentrations can be used. 

The cons are:

• The metallic substrate chosen may be reactive, unlike silver.
• Full interpretation of the data is difficult as the theory is only partially understood.

Non-Resonance Raman Spectroscopy

• Non-resonance Raman spectroscopy occurs when the radiation interacts with a molecule, which results in the polarization of electrons of the molecule. The enhancement in energy from the radiation excites the electrons to an unstable virtual state, which results in the immediate discontinuity of interaction. This leads to the scattering of radiation at a slightly different energy than the incident radiation.
• It is used for the analysis of water-containing samples because of water’s low polarizability. It can be used for analysis of samples having concentrations no lower than 0.1 M. 
• However, the signals are weak and can be easily overwhelmed by fluorescence signals.

Resonance Raman Spectroscopy

• The incident radiation is at a frequency near the frequency of the electronic transition of the molecule. This gives enough energy for the excitation of electrons to the higher electronic state. A tunable laser is preferred as there is no need to change the laser for the measurement of multiple samples, as each one may require different excitation wavelengths.
• It is useful in studying and authenticating artwork and artifacts. It is used to analyze concentrations of samples as low as 10^-8 M. It produces a spectrum with only a few Raman lines as it only augments Raman signals affiliated with chromophores in the analyte.
• Here also, fluorescence is a problem as the technique itself can induce fluorescence of molecules since there is a possibility of electrons returning from an excited state to ground state.

The table below shows the pros and cons of some variants of Raman spectroscopy:

Conclusion

We have come to know about different variants of Raman spectroscopy. Though all variants are not discussed, important types of Raman spectroscopy have been described. Some variants have become indispensable to advanced experiments. However, depending on the severity of the need, the variant is chosen and the Raman imaging process is executed.