Introduction To Raman Spectroscopy

Raman spectroscopy is a kind of molecular spectroscopy, which is observed as an inelastically scattered light, and authorizes the interrogation and identification of (phonon) vibrational states of molecules. Hence, Raman spectroscopy delivers as a priceless analytical tool for molecular fingerprinting and monitoring changes in the bond structure of molecules. Raman imaging and Raman analysis are also used to denote the Raman spectroscopy. Examples are formation of product, changes of states, stresses, strain, crystalline forms and crystallinity.

This article bestows the prologue of the Raman spectroscopy to the readers. 

Pros of Raman spectroscopy

In comparison to other vibrational spectroscopy techniques like NIR and FTIR, Raman spectroscopy offers some advantages as follows:

As contradictory to the light absorbed by a sample, the Raman effect attests itself in the light scattered off a sample, 

It demands no preparation of samples

It exhibits indifference to aqueous absorption bands

These characteristics eases the measurement of gas, liquids and solids not only directly, but also via transparent cases like plastic, quartz and glass.

Application of Raman spectra

As Raman spectroscopy is a complementary technique to FTIR spectroscopy, it is highly selective. This permits it to recognize and discriminate molecules and species, which are quite similar and examine minute changes in molecules.

Organic solvents like ethanol, acetone, dimethyl sulfoxide, toluene and ethyl acetate own slightly differing molecular structure with different functional groups. Hence, the Raman spectra of them are explicitly dissimilar as observed in the picture. 

Thus, using the libraries of Raman spectrum, it is smooth to see how effortlessly the Raman spectra is able to be utilized for material recognition and authentication.

Light interpretation

In classical wave interpretation, light is deemed as electromagnetic radiation, containing an electric field, which interacts with a molecule via its polarizability. Polarizability is ascertained by the ability of electron clouds to interact with an electric field. Soft molecules like benzene are inclined to be strong Raman scatterers, whereas hard molecules such as water have a tendency to be moderately weak Raman scatterers.

In quantum particle interpretation, light is depicted as a photon, striking the molecule and eventually scattering inelastically. The number of scattered photons is directly proportional to the size of the bond. Molecules with larger Pi bonds like benzene show a tendency to scatter a plenty of photons, whereas water molecules with smaller single bonds act as a weak Raman scatterer.

With the frequency directly proportional to the strength of the bond and inversely proportional to the reduced mass, the molecule vibrates in a cosine model. Each molecule has its own unique vibrational signatures determined by atoms in the molecule and characteristics of individual bonds. As the polarizability of a molecule is a function of displacement, the vibrational frequencies can be calculated via the Raman effect 

Outcomes of Scattering

When the incident photon excites the molecule into a virtual energy state, there are three different potential outcomes. 

1. The molecule can relax and back down to the ground state and emit a photon of equal energy to that of the incident photon. This is an elastic process and is referred to as Rayleigh scattering.
2. The molecule is able to get relaxed to a real phonon state and emit a photon with lesser energy than that of the incident photon. This is described as Stokes shifted Raman scattering.
3. When the molecule is already in an excited phonon state and excited to higher virtual state, then it relaxes back down to the ground state, emitting a photon with more energy than the incident photon. It is called anti-Stokes Raman scattering.

There is a significantly lower probability that a photon will be anti-Stokes scattered, considering most of the molecules are present in the ground state at room temperature. Consequently, most Raman measurements are performed considering only the Stokes shifted light.

There exists a linear relationship between the intensity of incident light and power of the scattered light. Besides, there is a relationship between the power of the scattered light and the inverse of the wavelength to the fourth power.

Laser Selection in Raman Spectroscopy Instrumentation

Typically, a laser is the best excitation source for Raman spectroscopy. Yet, all lasers are not suitable for Raman spectroscopy and hence, it is a necessity that the frequency of the laser is extremely stable and does not make hop, as it causes errors in the Raman shift. 

Utilization of clean and narrow bandwidth lasers is essential as the quality of the Raman peaks is explicitly influenced by the sharpness and stability of the excitation light source.

If the wavelength is shorter, the more powerful the Raman signal. Generally, the visible lasers are used only for inorganic molecules like carbon nanotubes as organic molecules can get fluorescence even though excited by longer wavelengths, overwhelming the signal in the Raman spectrum.

785 nm diode lasers are the industry standard as they reduce fluorescence to the maximal range without sacrificing spectral range or resolution. Samples suffering from strong fluorescence interference and are highly colored, are used with 1064 nm laser as maximal suppression of fluorescence occurs. The 532 nm laser is the best choice for measurement of Raman spectrum in inorganic molecules with increased sensitivity as fluorescence is not an issue.

Conclusion

A short definition of Raman spectroscopy has been given. Then, some pros of Raman spectroscopy have been listed. Uses of Raman spectra have been given. Then. light interpretation as classical wave interpretation and quantum particle interpretation have been given. Three potential outcomes during the interaction of incident photons and molecules are discussed. Reasons for neglecting the anti-Stokes phenomenon are provided. Finally, a short note on selection of lasers has been given.