Fine structure and collectivity of the levels of the pygmy dipole resonance in $^{208}$Pb in a self-consistent model

A novel fully self-consistent microscopic approach based on the energy density functional method is employed to calculate the fine structure of the pygmy dipole resonance in $^{208}$Pb, i.e., the energies and reduced probabilities of $E1$ transitions for the states with energies below $10$ MeV. The approach includes the random-phase approximation, quasiparticle-phonon interaction and the single-particle continuum. The theoretical results are compared to the available high-resolution data and found to agree with measured integral characteristics of the pygmy dipole resonance at energies above $5.7$ MeV. Residual spin-spin forces are quantified, and their contribution is found to be significant at both low and high energies. A recently proposed criterion is employed to analyze the collectivity of the $1^-$states in $^{208}$Pb.