Spectroscopy Series VOL. 5: Raman resonance

Greetings friends

First of all, I want to give you my best wishes this Christmas and again I would like to share with the whole community a new message related to spectroscopy, then I will present a brief summary of one of the techniques used in this field of science.

If you want to read about the previous deliveries click on the following links:

The Raman resonance is a technique that allows us information about the vibrations of the molecules, through it we can observe frequencies ranging from 1012 to 10 14 Hz which, as we know, is the infrared region of the electromagnetic spectrum. It is also often used to identify unknown substances.

^{Spectrum of an organic compound 19F NMR Creative Commons Attribution-ShareAlike License of Jkwchui}

We can explain in a simple way the following: the photon affects the sample to be characterized and the energy it generates causes it to be close to the energy of the electronic transition. The resonance allows it to reach a much greater intensity than the Raman dispersion, which results in a better visibility of some chemical compounds that have very low concentrations.

This technique allows us to observe some ways of vibration of crystalline networks at the same time, because it decreases the complexity of observing the resonant spectrum, its main advantage is the extension of the diffraction peaks of the analyzed compounds, since it increases the intensity of said peaks by a factor of about 106 which, as I have just mentioned, shows spectra with very low concentrations. With all this, it is a primordial technique to identify new phases in materials.

^{Argon-Ion Laser Creative Commons Attribution-ShareAlike License}

A transition is generated from the molecule to the material where the laser generates a wavelength that must exactly coincide with the electronic region. The lasers used for this technique are those of Argon and Krypton-ion since they generate an amplitude of the electronic transitions, where we can facilitate the observation of the Raman spectrum, however, you can also use old lasers because they can tune to match a resonance transition.

The vibration modes present a very important change during this effect where the force constant experiences an excitation in the electronic transition that brings with it an increase in the polarization and, consequently, in the intensity of the Raman effect, said increase is included in a factor from 10a> 100.00 and much more prominent in the π-π transitions.

^{Molecular energy levels and Raman effect Licensed CC BY-SA 3.0}

Instrumentation

The components used for Raman resonance is basically the same as that used in conventional Raman spectroscopy, we need a monochromatic light source, which in this case would be the laser device, which must have a near infrared emission wave and be visible to the ultraviolet spectrum. As mentioned above, the idea is that the wavelength emitted by the laser coincides with the electronic band of the material that is to be analyzed.

^{Nuclear Magnetic Resonance Spectrometer Licensed CC BY-SA 3.0 of Daniel Alexandre}

The light is scattered in several directions for a more efficient data collection, the difference is that this light is achieved at a very small angle by a lens that focuses the concentration of the area where you want to analyze the material, then collect all the scattered light which is then directed to the spectrograph and finally to the coupled charge detector where it records the Raman scattering data.

The collected dispersion must be focused on the spectrograph, the light must pass through a filter, which is where the wavelength interference occurs, then it is dispersed by a grid and focuses on the camera that allows controlling the area of incidence of the laser, which in turn has a mirror that can be controlled by means of a rotating screw. This camera focuses on the coupled charge detector, where the entire spectrum is recorded simultaneously and we can obtain several sweeps of the material in a small period of time, which means that by increasing the signal and noise of the spectrum through the average.

Applications

Raman resonance concentrates in the area of chemistry in the characterization of transition metal peroxide complexes, since it gives us important information on the mode of binding of the peroxide ligand.

^{Carbon nanotubes Pixabay}

In the same way this technique is perfect for studies in the field of condensed matter physics, specifically in obtaining nanoparticle spectra. The power of Raman spectroscopy for the characterization of carbon nanotubes was first demonstrated, in 1997, by Rao et al. who showed a dependency of the Raman spectra from SWNT beams in the Lasers energy excitation laser, due to a strong resonance effect between Lasers and the strong ones. And over the years there have been many more studies with carbon nanotubes.

^{Spectra of some medications Creative Commons Attribution-ShareAlike License. Autors: Brian A. Eckenrode, Edward G. Bartick, Scott D. Harvey, Mark E. Vucelick, Bob W. Wright, and Rebecca A. Huff}

Other studies such as Structural and Stereochemical Analysis for the characterization of chemical compounds with active nuclei. Identification and quantification of organic compounds, organometallic, etc. Impurity control Studies of dynamic systems and molecular physical parameters. Quality control in food. Diagnosis and molecular prognosis in clinic. Determination of metabolic profiles in biopsies and / or biofluids.

What are the advantages and disadvantages of Raman Resonance?

Comparison between resonance and non-resonance..

Raman spectroscopy has a great variety of techniques each one has advantages and disadvantages with respect to the other, each instrumentation is suitable or not for certain specific studies, for example, the Non-Resonance is considered the most appropriate to perform water studies due at low polarization. Both have the ability to analyze specimens in solid, liquid and gaseous state. Due to its great ability not to destroy the samples, it is perfect for analyzing very delicate materials.

In the Raman resonance spectroscopy, monochromatic light is used in infrared and ultraviolet regions. Generally a tunable laser is used for resonance and this could be said to be a great advantage for the characterization of materials using this technique, since only one laser is needed to perform the measurements of multiple samples, this allows the operator to perform analysis No need to change the laser. For example, if we have only one type of tunable laser in our laboratory, we can use any type of laser to improve the Raman signal, it should be noted that the laser must have a frequency close to the electronic transitions, otherwise we would not be characterizing of Raman resonance.

To complete the Raman spectroscopy of resonance is very sensitive compared to the non-resonance. It has the great ability to analyze specimens with very low concentrations. Raman resonance spectroscopy produces a spectrum with relatively few lines, that is, the technique only increases the Raman signals associated with the chromophores in the analyte. This makes the technique particularly useful for the analysis of larger molecules such as biomolecules.