What is Raman Scattering?

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- GoPhotonics

Aug 22, 2023

Raman scattering, also known as the Raman effect, is a phenomenon in which the interaction of incident light with a transparent medium generates scattered light. 

When light is scattered by molecules upon irradiation, most scattered light has the same frequency as incident light. This means that there is no energy change, it is called Rayleigh scattering. However, a fraction of the light has a different frequency due to the interaction between the oscillation of light and molecular vibration. This phenomenon, where light is scattered with a frequency change, is known as Raman scattering.

The frequency shift observed in the scattered photons qualifies it as a nonlinear scattering. It is also considered an inelastic scattering process due to the energy exchange that occurs between the incident photons and the molecules or crystals involved. Raman scattering is not limited to solid materials but can occur in liquids and gases as well. In the year 1928, C.V. Raman made a discovery of the inelastic scattering of photons by molecules, resulting in their excitation to higher energy levels known as Raman scattering. This effect offers valuable insights into the molecular and structural properties of materials.

When light interacts with a transparent optical medium, most of the incident photons propagate through the material unaffected, but a small fraction interacts with the molecules or vibrational modes of atoms within the crystal lattice of the medium,  resulting in vibrations in the crystal lattice or molecular bonds. These vibrations result in the excitation of the molecule to a  higher energy level or the relaxation of the molecule from a higher energy level to a lower energy level. The energy exchange between the photon and the molecule leads to non-instantaneous scattering of light with a shifted frequency. This frequency shift is known as the Raman shift and is typically measured in units of wavenumbers (cm-1). It depends on the characteristics of the incident light source and the rotational and vibrational properties of the scattered molecules.

When a photon from the monochromatic source interacts with a molecule or crystal,  the oscillating electromagnetic field of the photon causes the molecular electron cloud to polarize. As a result, the molecule enters a higher energy state, with the energy of the photon transferred to the molecule. This process can be seen as the temporary formation of a complex between the photon and the molecule, known as the virtual state. However, the virtual state is short-lived and unstable, causing the photon to be re-emitted almost instantly as scattered light.

In the majority of events, there is no energy change in the molecule, therefore light of the same frequency is re-emitted. This is called Rayleigh scattering. Rarely there is energy transfer between the molecule and scattered photon.

  • After being excited from a ground level, if an electron falls to a vibrational level, instead of the ground level, then the molecule absorbed a certain amount of energy from the photon, which results in light being emitted at a lower frequency than the incident light. This Raman scattering is called Stokes scattering. Since the population of the ground level is more compared to vibrational levels, Stokes scattering is the more common and dominant process in Raman spectroscopy.
  • If an electron is excited from a vibrational level, it reaches an excited state with higher energy. When the electron falls down to the ground level, the scattered photon has a higher frequency compared to the incident photon, which results in a shorter wavelength. This type of Raman scattering is called Anti-Stokes scattering.

Cascaded Raman Scattering

Cascaded Raman Scattering refers to a phenomenon where the intensity of the generated Stokes wave (resulting from Raman scattering) becomes sufficiently high that it can act as the pump for a subsequent Raman process. In the process of cascaded Raman scattering, the initial incident light interacts with the medium, undergoing Raman scattering to produce a lower-frequency Stokes wave. As the intensity of the incident light increases, the intensity of the Stokes wave increases. This generated Stoke wave can act as a new pump for another Raman scattering process, leading to the creation of additional Stokes waves with even lower frequencies. This cascading effect can continue, resulting in the generation of multiple Stokes waves at different frequencies.

The cascaded Raman scattering effect is particularly observed in some Raman lasers, where the high-intensity Stokes wave can stimulate further Raman scattering, resulting in the generation of additional Stokes orders.

The wave number of Stokes scattering can be given as:

Anti-Stokes scattering can be given as:

where ṽ0 is the wavenumber of the laser and ṽM is the wavenumber of the vibrational transition.

Applications of Raman Scattering

Raman scattering provides valuable information about the molecular and structural characteristics of materials. Raman spectroscopy, which is based on the Raman scattering phenomenon, is widely used in various fields such as chemistry, materials science, biology, and pharmaceuticals. It enables non-destructive and non-invasive analysis of samples, even in complex environments. Additionally, Raman scattering has applications in fiber optics, imaging, and sensing, where it can be used for chemical imaging, remote sensing, and detection of trace substances.