مطيافيه رامان Raman Spectroscopy
Raman spectroscopy is an analytical technique where scattered light is used to measure the vibrational
energy modes of a sample. It is named after the Indian physicist C. V. Raman who, together with his research partner K. S. Krishnan, was the first to observe Raman scattering in 1928.1 Raman spectroscopy can provide both chemical and structural information, as well as the identification of substances through their characteristic Raman ‘fingerprint’. Raman spectroscopy extracts this information through the detection of Raman scattering from the sample. *
?What is Raman Scattering
When light is scattered by molecule, the oscillating electromagnetic field of a photon induces a polarisation of the molecular electron cloud which leaves the molecule in a higher energy state with the energy of the photon transferred to the molecule. This can be considered as the formation of a very short-lived complex between the photon and molecule which is commonly called the virtual state of the molecule. The virtual state is not stable and the photon is reemitted almost immediately, as scattered light *
?What is Raman Spectroscopy
Fig. 1: Raman principle
Raman Spectroscopy is a non-destructive chemical analysis technique which provides detailed information about chemical structure, phase and polymorphy, crystallinity and molecular interactions. It is based upon the interaction of light with the chemical bonds within a material.
Raman is a light scattering technique, whereby a molecule scatters incident light from a high intensity laser light source. Most of the scattered light is at the same wavelength (or color) as the laser source and does not provide useful information – this is called Rayleigh Scatter. However a small amount of light (typically 0.0000001%) is scattered at different wavelengths (or colors), which depend on the chemical structure of the analyte – this is called Raman Scatter.
A Raman spectrum features a number of peaks, showing the intensity and wavelength position of the Raman scattered light. Each peak corresponds to a specific molecular bond vibration, including individual bonds such as C-C, C=C, N-O, C-H etc., and groups of bonds such as benzene ring breathing mode, polymer chain vibrations, lattice modes, etc. *
Information provided by Raman spectroscopy
Fig. 3: Raman spectra of ethanol and methanol, showing the significant spectral differences which allow the two liquids to be distinguished.
Raman spectroscopy probes the chemical structure of a material and provides information about:
- Chemical structure and identity
- Phase and polymorphism
- Intrinsic stress/strain
- Contamination and impurity
Typically a Raman spectrum is a distinct chemical fingerprint for a particular molecule or material, and can be used to very quickly identify the material, or distinguish it from others. Raman spectral libraries are often used for identification of a material based on its Raman spectrum – libraries containing thousands of
spectra are rapidly searched to find a match with the spectrum of the analyte
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