Low-lying component in strength distributions of weakly bound neutron-rich nuclei

We study the multipole response of nuclei for situations in which the neutron content becomes progressively rich. It is observed that the occurrence of weakly bound states towards the neutron drip line gives rise to low-lying concentrations of strength in the continuum which are not of a resonant character. These do not result from a redistribution of the expected high-lying response, but rather amount to a significant source of additional strength. The effect can be accounted for in terms of sum rules that grow — beyond a straightforward increase in the number of particles involved — because of the large spatial extent of the weakly bound orbitals.