Isovector and isoscalar dipole excitations in [9]Be and [10]Be studied with antisymmetrized molecular dynamics

Abstract

Isovector and isoscalar dipole excitations in [9]Be and [10]Be are investigated in the framework of antisymmetrized molecular dynamics, in which angular-momentum and parity projections are performed. In the present method, 1p-1h excitation modes built on the ground state and a large amplitude α-cluster mode are taken into account. The isovector giant dipole resonance (GDR) in E>20 MeV shows the two-peak structure, which is understood from the dipole excitation in the 2α core part with the prolate deformation. Because of valence neutron modes against the 2α core, low-energy E1 resonances appear in E<20 MeV, exhausting about 20% of the Thomas-Reiche-Kuhn sum rule and 10% of the calculated energy-weighted sum. The dipole resonance at E∼15 MeV in [10]Be can be interpreted as the parity partner of the ground state having a [6]He+α structure and has remarkable E1 strength because of the coherent contribution of two valence neutrons. The isoscalar dipole strength for some low-energy resonances is significantly enhanced by the coupling with the α-cluster mode. For the E1 strength of [9]Be, the calculation overestimates the energy-weighted sum (EWS) in the low-energy (E<20 MeV) and GDR (20<E<50 MeV) regions by a factor of 1.6 and underestimates the width of the GDR, whereas it reasonably describes the GDR energy and also the ratio of the EWS in the low-energy region to that of the GDR region.