Understanding Raman Spectroscopy - Principles and Theory

Raman Spectroscopy

Raman spectroscopy is one of the most widespread spectroscopic techniques and is used in sophisticated research laboratories. It is complementary to infrared absorption spectroscopy. It involves radiating a monochromatic light source or laser on the sample and detection of the scattered light. Raman imaging and Raman spectroscopy imaging are the other terms often used for the technique.

Stokes Shift

Without interaction, the majority of the scattered light will pass through the sample. So, the detector receives energy, which is of the same frequency as the excitation source. It is known as elastic scattering. It is otherwise called Rayleigh scattering. A very meager amount of the scattered light (1 in 10⁷) is shifted in energy from the frequency of the laser. This phenomenon is called the Stokes shift or Raman shift. 

Anti-Stokes Shift

The anti-Stokes shifted Raman energy is feeble than Stokes shifted Raman energy at room temperature. So they are ignored and removed by filters. 

Raman Scattering

The scattering happens because of the interactions between the incident electromagnetic waves and the vibrational energy levels of the molecular samples. The interaction is perceived as a disturbance to the electric field of the molecule.

Raman spectroscopy is not only limited to intramolecular vibrations, but also to crystal lattice vibrations and other motions of extended solids. The Raman spectra analysis is much significant in the fields of mineralogy and geochemistry. 

Molecular Polarizability

Raman spectroscopy is explained by the interaction of the electromagnetic field with the bonds of the molecule. The dipole moment induced by the external electric field in a molecule is directly proportional to the electric field.

P = alpha*E

Where alpha is proportionality constant, represents the polarizability of the molecule. The ease with which the electron cloud around the molecule can be distorted is measured by the polarizability of the molecule. The induced dipole scatters or emits light at the frequency of the incident light. Within the bond of the molecules, there is a change in polarizability, which gives rise to Raman scattering. The intensity of the scattering is directly proportional to the square of the induced dipole moment. 

Raman Spectroscopy with respect to Molecular Polarizability

When the vibration is not able to change the polarizability much, the value of the polarizability equals near-zero, leading to the lower intensity of the Raman spectrum. The vibrations of the highly polar groups like O-H bonds are generally weak. So, when an external electric field is incapable to induce a greater change in the dipole moment, and bending or stretching of bonds will not happen, leading to a weak Raman signal. 

Strong Raman scatterers are groups with distributed electron clouds like carbon-carbon double bonds. In an external electric field, the pi-electron cloud of the double bond is easily distorted. Stretching or bending the bond alters the distribution of electron density consequently and generates a greater change in induced dipole moment. 

For the molecules to be polarized, the incident photon energy excites vibrational modes of the molecules, providing scattered photons that are diminished in energy equal to the number of vibrational transition energies. This gives rise to the peaks in a Raman spectrum. The number of peaks is linked to the number of degrees of freedom a molecule consists of.

To be Raman active, a molecule should possess a change in polarizability. Polarizability, in other words, is the relative tendency of electron clouds to be distorted from the normal shape.

Raman Spectrum

The plot of the intensity of the shifted light against the frequency defines the Raman spectrum of the sample. Usually, Raman spectra are plotted against laser frequency in such a way that the Rayleigh band exists at 0 cm^-1. On this scale, the positions of the band exist at frequencies. These correspond to the levels of energy of different functional group vibrations. Thus, the Raman spectrum can be interpreted as equivalent to the infrared absorption spectrum. 

Pros of Raman Spectroscopy

The following points list the advantages in the use of Raman spectroscopy:

• For sample preparation, little to no effort is needed in most cases. The sample can be readily placed in the holder position and the spectrum can be simply retrieved.
• Water itself can be used as a solvent, though water is a weak scatterer. Even complex samples are used with water and there are no special accessories needed for measurement in aqueous solutions.
• There is no need for nitrogen purging in the optical bench as water and carbon dioxide vapors are weaker in the scattering process
• The sample holders are inexpensive and cheap. Glass holders are used in most cases.
• Raman spectra look cleaner than mid - IR spectra. The Raman lines are narrower and combination and overtone bands are weak.
• The conventional spectral range reached well below 400  cm^-1, thus fabricating the technique, ideal for both inorganic and organic species. Thus it is suitable for the investigation of a wider range of molecules.
• Symmetric linkages, which are weak in the infrared spectrum like carbon-carbon double bond, carbon-sulfur single bond, and sulfur-sulfur single bond can be easily investigated through Raman spectroscopy.

Cons of Raman Spectroscopy

Raman spectroscopy suffers from a few limitations and they are mentioned as follows:

• The sensitivity of the detector is supreme as the intensities of Raman spectra are lower.
• Compared to the mid-IR spectrum, the instrumentation is high-priced.
• When the setting of the power is too high, the laser of the Raman spectrometer can destroy some sections of the sample
• A major concern is that the laser causes fluorescence in some samples.

Conclusion

A precise introduction to Raman spectroscopy has been given initially. Then, the Stokes shift and anti-Stokes shift have been briefed. Raman scattering has been explained. In addition, molecular polarizability and its relation to the Raman spectrum have been discussed elaborately The Raman spectrum has been defined. Further, the pros and cons of Raman spectroscopy have been listed.