Self-consistent Multilevel PRD

After having recalled what are the physical components of frequency partial redistribution (PRD), this paper presents a new multilevel theoretical formalism based on non-markovian master equation theory for describing the atom -radiation interaction. As an application, this paper present final results of a modeling, based on this theory, of the second solar spectrum of the Na I D lines, which is the spectrum of the linear polarisation formed by scattering and observed close to the solar limb.