Rayleigh and Raman Scattering

Abstract

The statistical multi-level approach of the preceding chapter that is based on the density matrix formalism provides a foundation for physical understanding and for the treatment of quite general problems. It can account for the effects of coherences of any atomic level, although the effects of collisions are included in a phenomenological and incomplete way (this will be improved on in the next chapter). In practice, however, coherence phenomena play a significant role only in connection with scattering processes (Rayleigh, resonant, Raman, and fluorescent scattering). The dominant polarization effects come from the coherences in the intermediate, excited states of the scattering process, while lower-state coherences only result in minor perturbations of the scattering polarization, effects that would be very hard to measure in astrophysical situations. It is therefore a good approximation to neglect atomic polarization and coherences (phase relations between the magnetic substates) of the initial level of the scattering process. This greatly simplifies the problem, since the scattering polarization may be calculated without solving the polarized statistical equilibrium equations.