In recent years, Raman spectroscopy has been applied in various fields such as jewelry identification, biology, medicine, etc., and has great value in pure qualitative analysis, highly quantitative analysis and determination of molecular structure. Therefore, many laboratories are equipped with Raman spectrometers at this stage. Today, LANScientific will share some practical knowledge of Raman spectroscopy with you.
1. Raman's discovery
In the summer of 1921, on the "Nakunda" passenger ship sailing in the Mediterranean, an Indian scholar observed the sea surface with a simple optical instrument on the deck. He was fascinated by the deep blue color of the sea, and he was bent on exploring the source of the color of the sea.
This scholar is the famous scientist Raman. Through experimental observation and analysis during his voyage, he found that the maximum value of the sea water spectrum is more blue than the maximum value of the sky spectrum. It can be seen that the color of the sea water is not caused by the color of the sky, but a property of the sea water itself. Raman thinks this is due to the scattering of light by water molecules!
2. Raman's research
Raman decided to further explore the theory and laws of this phenomenon. Inspired by American AH Compton's discovery of "the phenomenon that X-rays are scattered by matter, the wavelength becomes longer", on the afternoon of February 28, 1928, Raman used monochromatic light as the light source to conduct a decisive experiment: he used monochromatic light as the light source. Visual inspection of the spectroscope to look at the scattered light reveals that there are more than two sharp bright lines in the blue and green areas. Each incident spectral line has a corresponding variable scattering line. This newly discovered phenomenon is known as the Raman effect.
3. The principle of Raman spectroscopy
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 Raman spectroscopy.
The Raman effect is the result of the interaction of photons with optical branch phonons. When light irradiates a substance, elastic scattering and inelastic scattering occur. The scattered light of elastic scattering has the same wavelength as the excitation light, and the scattered light of inelastic scattering has components longer and shorter than the wavelength of the excitation light, which are collectively referred to as the Raman effect. , Raman spectroscopy came into being.
There are three kinds of relative interactions between matter and light: 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 (XRF), 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.), Raman scattering spectroscopy (Raman).
4. Comparison of Raman Spectroscopy and Infrared Spectroscopy
Both Raman spectroscopy and infrared spectroscopy can obtain information about various normal vibrational frequencies and relevant vibrational energy levels inside the molecule, which can be used to identify the functional groups present in the molecule. However, the principles and mechanisms of the two are different. In molecular structure analysis, Raman spectroscopy and infrared spectroscopy complement each other, and some information that cannot be detected in infrared spectroscopy can be well represented in Raman spectroscopy.
Infrared spectroscopy focuses on detecting groups and is suitable for polar bonds, mostly used to detect organic substances. Raman spectroscopy detects molecular skeletons and is suitable for non-polar bonds, both organic and inorganic.