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Introduction to Raman spectroscopy
When an electromagnetic radiation passes through a transparent medium, existing species scatter part of the beam in all directions. In 1928, C. V. Raman discovered that the wavelength corresponding to a small fraction of the radiation scattered by certain molecules was different from the wavelength of the original radiation (ie, inelastic scattering occurs). Wavelengths vary depending on the molecular structure of the compounds. Raman spectroscopy is based on the analysis of these differences to determine the molecular structure of different compounds .
Scattering is a physical process in which a type of radiation such as light, sound, or even a beam of moving particles (such as ions, electrons, etc.) collides with particles or different surfaces in a direct path in which He is moving and deviates and is forced to move in one or more other directions (Figure 1). Scattering usually occurs in all directions .
Due to the collision of light with matter, we will have two types of scattering according to the wavelength of the scattered radiation:
- Rayleigh scattering is caused by particles that are much smaller than the wavelength of the radiation. Due to this type of scattering, the radiation wavelength does not change and is also classified as elastic scattering. The most obvious example of this type of scattering is the blue color of the sky, which occurs due to the scattering of shorter wavelengths in the visible spectrum.
- Raman scattering, in which the initial wavelength changes due to the transfer of energy between the photons and the matter molecules, and the wavelength increases due to the loss of energy, or the wavelength decreases due to the capture of energy. Finds. The magnitude of these energy changes (whether decreasing or increasing) is proportional to the frequency of the molecular vibrations of the light scattering species. Raman scattering will be divided into two general categories. The first group, which has a longer wavelength (less energy) than the original radiation, is called Stokes, and the second group, which has a shorter wavelength (more energy) than the original radiation, is known as anti-Stokes. 2].
- Spectrum Raman
Figure 2 shows a part of a Raman spectrum for CCl4 species in which the sample is irradiated with a laser source with a wavelength of 488 nm. In a horizontal axis Raman spectrum, generally in terms of the scattered radiation wave number (ῡ) or, as shown below, in terms of the changes made in the scattered beam wave number (ῡ2) relative to the source radiation wave number (ῡ1), ie in terms of the wave number changes ( 2) (which in practice indicates the scatter created in a specific wave number). While the vertical axis shows the intensity of the peaks in relative terms. Note that the relationship between the wave number of a radiation and its wavelength (λ), frequency (υ) and energy (E) is as follows and has a unit of cm-1:
ῡ = 1 / λ and ῡ = υ / c
E = hυ = hcῡ
As can be seen in the figure below, the Stokes lines are more intense, which is justified by their higher probability of occurrence, as photons are more likely to lose energy due to contact with the material environment than to receive them. Is energy. Another thing to keep in mind is that the amount of Raman Shifts (written numerically above the peaks) is independent of the laser wavelength used to excite the sample. It should also be noted that Riley scattering is located exactly at the wavelength equal to the source wavelength, its displacement rate is zero and its intensity is much higher than the Stokes and anti-Stokes lines .
Before continuing the discussion, it is necessary to point out that due to the continuity of the material, in order to better understand the following sections, it is better to first read the article on infrared spectroscopy. Below, due to the great similarity and complementarity of infrared and Raman spectroscopy techniques, a comparison is made on the differences.
- Investigation of differences between Raman technique and infrared spectroscopy
Studies have shown that shifts in the wavelength (wave number) of the source due to Raman scattering are in the infrared spectral range. In simpler terms, the difference between the energy of the source radiation and the scattered radiation is equal to the energy of the waves in the middle infrared range (see the article Infrared Spectroscopy). As mentioned in the article on infrared spectroscopy, this amount of energy is sufficient only for transitions between molecular vibrational levels of molecules (Molecular Vibrational Levels), and in this respect two methods are similar to each other. The Raman scattering spectrum and the infrared spectrum for a particular species are often very similar. There are many similarities between the two methods, but it should be noted that despite these similarities, the two techniques are different in principle and theory in that they are usually used as a complement to each other. In the paper introducing the infrared spectroscopy method, it is mentioned that one of the necessary conditions for a particular bond to be active in infrared spectroscopy is to cause a net change in dipole moment due to the absorption of radiation (Refer to the main article).