Fundamentals
Raman effect
The Raman effect
The scattering of light by molecules or crystal lattices is a very weak effect. If monochromatic light is scattered by molecules or crystal lattices spectral analysis shows an intense spectral line matching the wavelength of the light source. Additionally, more weaker lines are observed at wavelengths which are shifted compared to the wavelength of the light source. These lines are called Raman lines (after the Indian physicist Chandrasekhara Venkata Raman). Although these lines had already been predicted theoretically, Raman was the first who experimentally confirmed them in 1928.
The interaction between matter and light can be interpreted as a collision between a vibrating molecule or lattice and an incident photon. There are three possibilities:
1. If the collision is elastic, the energy of the photon (green arrow) as well as the energy of the molecule do not change after the collision. The elastic scattering of the photon is called Rayleigh scattering.
The inelastic collision between a photon and a molecule is known as the Raman effect. The energy difference between the inelastic scattered photons and the incident photons is exactly the difference between two energy levels of a molecular vibration.
2. If the vibrational energy of the molecule is increased after the collision, the energy of the scattered photons is decreased for the same amount and, therefore, can be detected at longer wavelengths (red arrow). The respective spectral lines are called Stokes-shifted.
3. If the vibrational energy of the molecule is decreased after the collision, the energy of the scattered photons is increased for the same amount and, therefore, can be detected at shorter wavelengths (blue arrow). The respective spectral lines are called anti-Stokes-shifted. This is only possible if the molecule is an excited vibrational state before the collision.
