Energy-dependence of skin-mode fraction in E1 excitations of neutron-rich nuclei

We have extensively investigated characters of the low-energy E 1 strengths in N > Z nuclei, by analyzing the transition densities obtained by the HF+RPA calculations with several effective interactions. Crossover behavior has been confirmed, from the skin mode at low energy to the pn mode at higher energy. Decomposing the E 1 strengths into the skin-mode, pn -mode and interference fractions, we show that the ratio of the skin-mode strength to the full strength may be regarded as a generic function of the excitation energy, insensitive to nuclides and effective interactions, particularly beyond Ni.