Figure 15.4 — Energy levels schemes for different scattering processes Figure 15.4 — Energy levels schemes for different scattering processes. (a) In Rayleigh scattering, the emitted photon has the same frequency as the incident one. (b) For Stokes emission, the scattered photon has a frequency lower than the incident one. (c) In anti-Stokes processes, the scattered photon has a frequency higher than the one of the incident light. (d) In coherent anti-Stokes Raman scattering, a four-wave mixing process takes place and resonantly excites the emission of anti-Stokes photons.