Polarization Charge of Particles near Threshold due to the Coupling to Shape Oscillations

It is shown that the isoscalar strength in the energy region just above the low-energy threshold, which is created by exciting particles to the continuum in nuclei far from β-stability lines, can be reduced by the attractive coupling to isoscalar shape oscillations. This is in contrast to the well-known fact that in β-stable nuclei low-lying isoscalar particle-hole strengths are always increased by the attractive isoscalar coupling, which leads to an appreciable amount of positive polarization charge. On the other hand, the core-polarization effect on bound particles with ℓ = 0 and 1 is always positive, but vanishes as the binding energy approaches to zero.