Crossover from Skin-Mode to Proton-Neutron-Mode in E1 Excitations of Neutron-Rich Nuclei

Character of the low-energy E1 excitations is investigated by analyzing transition densities obtained from the RPA calculations in the doubly-magic nuclei. We propose a decomposition method of the E1 excitations into the pn mode (i.e. oscillation between protons and neutrons) and the skin mode (i.e. oscillation of the neutron skin against the inner core) via the transition densities, by which their mixing is handled in a straightforward manner. Crossover behavior of the E1 excitations is found, from the skin mode at low energy to the pn mode at higher energy. The ratio of the skin-mode strength to the full strength turns out to be insensitive to nuclide and to effective interactions in the energy region of the crossover. Depending on the excitation energy, the observed low-energy E1 excitations are not necessarily dominated by the skin mode, as exemplified for Zr.