α-cluster structures and dipole excitations in medium-heavy nuclei

Continuum strength function of the isoscalar dipole (IS1) transition for ⁴⁴Ti → α + ⁴⁰Ca is investigated on the basis of the orthogonality condition model (OCM) under the absorbing boundary condition (ABC). The nuclear potentials are constructed from the double folding procedure with the effective nucleon-nucleon interaction of the density-dependent Michigan three range Yukawa (DDM3Y). The OCM + ABC calculation predicts that the enhancement in the IS1 strength induced by the excitation of the α – ⁴⁰Ca relative motion occurs at the lower excitation energy of E_x ≤ 10 MeV. The IS1 strength is larger than the strength by a single nucleon excitation. The low-lying enhancement in the E1 strength for ¹³⁵Cs → α + ¹³¹I is also discussed. (author)