Systematics of the bimodal isoscalar giant dipole resonance

The systematic behavior of the isoscalar giant dipole resonance (ISGDR) in Zr-90, Sn-116, S-144 m, and Pb-208 is studied with inelastic a scattering at E-alpha=386 MeV. Multipole-decomposition analysis is applied to extract the excitation strengths of giant resonances from the (alpha, alpha') differential cross sections at theta(lab)= 0.64degrees-13.5degrees. The bimodal structure of the ISGDR is discussed and compared with recent theoretical results from Hartree-Fock + random-phase-approximation calculations.