Raman scattering

Raman scattering is a form of inelastic scattering of light or other electromagnetic radiation by electrons, atoms, or molecules. Raman scattering can occur when light enters gases, plasmas as well as crystalline solids (phonon Raman scattering), and is due to the interaction of photons with the relevant particles of the medium involved. Such interactions concern, for example, rotational and vibrational energy in molecules or energy quanta of lattice vibration (optical or acoustic) phonons, in solids. In molecules or crystals, energy transfer can occur between the exciting photon and the excited matter, so that the rotational and vibrational energy of the involved molecule or the vibrational energy in the crystal lattice changes - i.e. there is a transition of the molecule from one energy level to another.

The term "inelastic" describes the fact that this type of scattering changes the kinetic energy of the particles involved. So there is an energy transfer between the radiant energy and the scattering medium, where the scattered light has a higher or lower frequency than the incident light beam. So there is an energy transfer between the radiant energy and the scattering medium, where the scattered light has a higher or lower frequency than the incident light beam. The respective frequency is specific for the scattering atom or molecule. If the frequency of the exciting photon is resonant with an electronic transition in the molecule or crystal, the scattering efficiency is increased by two to three orders of magnitude (resonance Raman effect). In the case of such inelastic scattering, the energy balance causes a frequency shift of the scattered light. Both directions of energy transfer are possible.

Raman scattering of molecules is characterized by a very small scattering cross-section. Consequently, a rather high concentration of molecules is needed to obtain detectable signals. Therefore, Raman spectra for single molecules are not possible. However, if the molecule is close to a metallic surface (e.g., made of silver, copper, or gold), this can extremely amplify the Raman signal. This so-called surface-enhanced Raman scattering (SERS) is used, for example, in surface-enhanced Raman spectroscopy and surface-enhanced resonance Raman spectroscopy (SERRS).

In the quantum mechanical model, the Raman effect can be described as a two-photon transition between quantized energy levels. Thus, Raman control occurs when photons interact with a molecule and the molecule is placed in a virtual higher energy state. This higher energy state can result in a few different outcomes, such as the molecule relaxing to a new vibrational energy state. Thus, a photon with a different level of energy is created. In this case, the difference between the energy of the incident photon and the energy of the scattered photon is called the Raman shift. When the interaction of the molecule with light leads to a shift in the electron cloud around the molecule, this also changes its polarizability.

In plasma physics, the Raman effect describes scattering by plasma waves, in which the light amplifies the plasma wave during the scattering process and heats up the plasma (Raman instability).

In solid-state physics, spontaneous Raman processes play an important role in the determination of vibrational spectra.

In contrast to fluorescence, Raman scattering is not a resonance phenomenon, but the scattering occurs here (as in Rayleigh scattering) via virtual levels. Raman scattering consequently also occurs for photon energy levels outside of resonance to an atomic or molecular electronic transition.

Note: The most important difference between Rayleigh and Raman scattering is the elasticity of Rayleigh scattering - whereas Raman scattering is inelastic. In (elastic) Rayleigh scattering, the kinetic energy of the random particles of the system in which the scattering occurs remains the same. Consequently, the frequency of the incident light coincides with that of the scattered light. On the other hand, when (inelastic) Raman scattering occurs, the kinetic energy of the random particles changes. Consequently, the incident light has a different frequency than the light scattered by Raman scattering.

Applications:‍

• Raman spectroscopy measures material properties such as crystallinity, crystal orientation, composition, strain, temperature, doping, etc. In this process, the matter to be examined is exposed to (laser) light from monochromatic excitation sources with longer wavelengths. In the spectrum of the light scattered by the sample, in addition to the irradiated frequency (Rayleigh scattering), other frequencies are observed that allow conclusions to be drawn about changes in the polarizability of the molecular bonds of the sample. Using Raman spectroscopy, the vibrational signature of a molecule can be detected down to the nanogram range to understand how the molecule is composed and how it interacts with other molecules in its environment.
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• Raman scattering measurement helps in the observation of crystallization processes, polymerization reactions, hydrogenation reactions, synthesis reactions, chemical synthesis, biocatalysis, and enzymatic catalysis. It is also used to identify polymorphic forms and is applied in flow chemistry and bioprocess monitoring.
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• In distributed temperature sensing (DTS), Raman scattering and its temperature dependence in optical fibers are utilized as linear sensors.

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