Question

Describe in words that what happens to a photon and its energy/ wavelength in a Raman experiment from the time it is emitted by a laser to the time when it detected as a vibrational anti-strokes line.

Answer 1

When a beam of light falls on a molecule photons are absorbed and scattered. Mostly the scattering is elastic and the scattered photons have exactly the same wavelength as the incident photons. In the scattering process electrons are excited to higher energy level due to collision with photon. These electron fall back to ground state by emitting a photon which has the same wavelength as that of incident photon. This is known as Rayleigh scattering

But sometimes inelastic collision may also take place. In Raman scattering, there is a change in the wavelength of the scattered photon.

It may happen that after the inelastic collision the electron may fall to a higher state (state having more energy than the ground state). since electron still has more energy than it initially had, it results in the emission of a photon having lower energy than the incident photon. These photons give Stokes lines.

The third scenario can be a collision with photon when the electron is already in an excited state (as when at high temperature). In this case, the energy radiated by an electron when returning to the ground state is the sum of the pre-excitation energy and the energy absorbed from the incident photon. Such photons which have higher energy or lower wavelength than the incident photons produce Anti-Stokes lines. Thus, anti-Stokes lines are always of shorter wavelength than that of the light that produces them