Introduction to the Basics of Raman Spectra
What is Raman Spectra
Raman spectra, named after Indian scientist C.V. Raman, is a method of molecular structure detection. Raman spectroscopy is a scattering spectrum, which is analyzed through a scattering spectrum with a frequency different from the incident light to obtain information on molecular vibration and rotation. The abscissa represents the Raman frequency shift, and the ordinate represents the Raman intensity, which is complementary to the infrared spectrum and can be used to analyze the relevant information of the intermolecular bond energy.
Principles of Raman Spectra
Raman effect: originates from molecular vibration (and lattice vibration) and rotation, so the knowledge of molecular vibration energy level (lattice vibration energy level) and rotational energy level structure can be obtained from the Raman spectrum.
The Raman effect is the result of the interaction between photons and optical phonons. Light irradiates the material to cause elastic scattering and inelastic scattering. The scattered light of elastic scattering is the same component as the excitation light wavelength, and the scattered light of inelastic scattering has longer and shorter components than the excitation light wavelength, collectively called the Raman effect . The relative effects of matter and light are divided into three types: reflection, scattering and transmission. According to these three situations, the corresponding spectral detection methods are derived: emission spectroscopy (atomic emission spectroscopy (AES), atomic fluorescence spectroscopy (AFS), X-ray fluorescence spectroscopy (XFS), molecular fluorescence spectroscopy (MFS), etc.) , Absorption spectroscopy (ultraviolet-visible light method (UV-Vis), atomic absorption spectroscopy (AAS), red appearance spectroscopy (IR), nuclear magnetic resonance (NMR), etc.), combined scattering spectroscopy (Raman scattering spectroscopy (Raman)). Raman spectroscopy came into being.
The abscissa of the Raman spectrum is called the Raman frequency shift. Raman scattering is divided into Stokes scattering and anti-Stokes scattering. The usual Raman experiment detects Stokes scattering. The difference between the frequency of Raman scattered light and Rayleigh light is the Raman frequency Shift, its calculation formula is Δν=| ν 0 – ν s |, that is, the difference between the scattered light frequency and the excitation light frequency. Δv depends on the change of molecular vibrational energy level, so it is characteristic, and the Raman spectrum has nothing to do with the wavelength of incident light, which is suitable for the analysis of molecular structure.
The History of the Development of the Raman Effect
In 1922, Smekal predicted that the frequency and direction of the new spectral lines would change.
In 1928, he observed a special spectrum of scattering in gases and liquids, and therefore won the Nobel Prize in Physics in 1930. In the same year, Mandieli Stam and Landsberger observed Raman scattering in quartz. From 1928 to 1940, it received widespread attention and even became the main method of studying molecular structure at that time.
But between 1940 and 1960, the status of Raman spectroscopy plummeted. The main reason is that the Raman effect is too weak. At that time, the requirements for sample testing were relatively strict, and the application of Raman spectroscopy declined.
After 1960, with the development of laser technology, laser became an ideal light source for Raman spectroscopy. With continuous improvement, Raman spectroscopy has been widely used, and more and more researchers have paid attention to it.
The Status of Raman Spectra
As Raman spectroscopy has the advantages of non-destructive, convenient, fast and high stability, it is currently used in food safety, biomedicine, molecular structure research, chemical process, biochemistry, archaeology and cultural relic identification, public security and legal sample analysis, and anti-terrorism technology It is more and more widely used in other industries.