What is Raman scattering?

We explained the principle of Raman effect and C V. How Professor Raman discovered the Raman effect.
Scientists use Raman spectroscopy to understand the chemical composition and structure of substances. We focus a single wavelength laser onto the sample. A small amount of light will interact with the chemical bonds of substances and change color during scattering. We can use a Raman spectrometer to measure inelastic scattered light and obtain relevant information about the sample. We also introduced the various components of a micro Raman spectrometer and the importance of each component for collecting good spectra.
What is spectroscopic technology?
We use spectroscopic techniques to measure the color and relative intensity of light interacting with matter. Spectral technology can tell us the chemical composition and physical or electronic structure of substances.
Light interacts with matter in different ways, transmitting through certain substances while reflecting or scattering on others. This interaction is influenced by the color (wavelength) of matter and light.
The parts of the visible spectrum that enter our eyes determine which colors we perceive. For example, if a substance absorbs the red part of its spectrum, it may appear blue. Only the blue part of the visible spectrum is reflected or scattered into our eyes.
Several basic phenomena that occur during the interaction between light and matter.
Who discovered Raman spectroscopy?
The Raman scattering process is named after its discoverer, the renowned Indian physicist and Sir Chandrasekhara Venkata Raman. C. Professor V. Raman and his student K S. Krishnan revealed that color changes occur when light passes through transparent materials. Light changes color and energy by interacting with molecular vibrations. This is the inelastic Raman scattering process. At that time, other scientists regarded the Raman effect as one of the most convincing proofs of quantum theory. C. Professor V. Raman was awarded the Nobel Prize in Physics in 1930 for this great discovery.
C. Professor V. Raman discovered the Raman effect in 1928. However, it was not until several decades later that advances in lasers, detectors, and computing technology facilitated the development of efficient Raman systems. Raman spectroscopy is now an essential tool in laboratories and manufacturing fields.
How to detect Raman effect?
You can use a Raman spectrometer to measure the Raman effect. The first step is to irradiate the sample with monochromatic light (such as laser). If you shine blue light on a substance, you may think you will only see the blue light reflected by it. The energy of most scattered light remains constant ("Rayleigh scattering").
Only about one millionth of the scattered light will undergo Raman scattering. Using a Raman spectrometer, you can detect Raman scattering light that changes in color and frequency. During the scattering process, light interacts with molecular vibrations, resulting in a change in frequency. Raman scattering occurs because photons (particles of light) exchange some energy with molecular vibrations in matter.
How does a Raman spectrometer measure vibration modes?
Raman spectrometer measures the energy difference between vibration modes by analyzing scattered light. When photons act on molecules, causing their electron cloud to polarize and temporarily elevate to a "virtual" energy level state, scattering phenomena occur. If photons change their energy during scattering, Raman scattering will occur. The reason for this phenomenon is that stimulated molecules transition to a vibrational energy level that is higher or lower in energy compared to their original state through an energy relaxation process.
Raman scattering is inelastic because photons change their energy by interacting with molecular vibrational energy levels. When scattered light loses energy, Raman scattering is called "Stokes". When scattered light gains energy, Raman scattering is called "anti Stokes". When a molecule temporarily transitions from its ground state to a virtual energy level due to energy absorption, and then falls to a vibrational energy level with energy higher than the initial ground state, Stokes Raman scattering phenomenon occurs. When a molecule is in a vibrational excited state, it can absorb photons and transition to a higher virtual energy state, after which the molecule releases energy and falls back from the virtual state to the ground state. During this process, anti Stokes Raman scattering occurs. We rarely use anti Stokes Raman light because its intensity is not as strong as Stokes Raman light. However, it does represent the equivalent vibrational information of the molecule.
On the contrary, when the molecule returns to the ground state, Rayleigh scattering occurs. It releases photons with the same energy as the incident photons. Therefore, the Rayleigh scattered light has the same frequency and color as the incident light. The intensity of Rayleigh scattering light is about 107 times that of Raman scattering light. Modern spectrometers use efficient filters to remove Rayleigh scattering light for successful detection of Raman scattering.
The mechanism of Raman scattering is similar to infrared (IR) absorption spectroscopy, but different selection rules apply. During the vibration process, it is necessary to change the molecular polarizability in order for Raman scattering to occur. Some vibrations can be seen in Raman spectra, but not in infrared spectra, and vice versa. For example, unlike infrared absorption spectroscopy, Raman spectroscopy can analyze carbon bonds in diamond.
What can Raman shift tell you?
Raman shift is the energy difference between the incident laser and the scattered light. The change in energy depends on the vibration frequency of atoms in the molecule. By studying molecular vibrations, we can discover the chemical and structural composition of substances.


If the Raman frequency shift or energy change is significant, it indicates that the molecular vibration frequency is very high. This is because light atoms are bound together through strong bonds. On the contrary, if the Raman frequency shift or energy change is small, it indicates that the molecular vibration frequency is very low. This is because heavy atoms are bound together through weak bonds.
Components of Microscopic Raman Spectrometer
The front end of a typical micro Raman spectrometer is an optical microscope. It is connected to the excitation laser, Rayleigh filter, spectrometer, and detector. The Raman effect is very weak; Only about one millionth of scattered light will undergo color changes due to frequency shift. This effect is too weak to be observed with the naked eye, so we need to use a high-sensitivity Raman spectrometer to observe this scattered light.