Configurational splitting of the giant dipole resonance in atomic nuclei

The formation of the giant dipole resonance (GDR) in light (A $<$ 40) atomic nuclei is studied. It is shown that the main feature of the GDR in these nuclei is configurational splitting, which is caused by the sharp drop of the deep 0s and 1p shells in 1p- and 2s, 2d-shell nuclei, respectively, and by the approximate spin-isospin SU$_4$ symmetry (1p-shell nuclei). As a result of this a single dipole state is not formed in light nuclei and the GDR is “spread” over groups of levels in different regions of the excitation energy. The role of this phenomenon in the system of modern ideas about the formation of collective states of nuclei is indicated and problems for further experimental and theoretical study are formulated.