In order to study isovector dipole response of neutron-rich nuclei, we have applied a time-dependent method of antisymmetrized molecular dynamics. The dipole resonances in Be, B and C isotopes have been investigated. In $^{ 10}$Be, $^{ 15}$B, $^{ 16}$C, collective modes of the vibration between a core and valence neutrons cause soft resonances at the excitation energy $E_x=10-15$ MeV below the giant dipole resonance(GDR). In $^{ 16}$C, we found that a remarkable peak at $E_x=14$ MeV corresponds to coherent motion of four valence neutrons against a $^{ 12}$C core, while the GDR arises from the core vibration in the $E_x >20$ MeV region. In $^{ 17}$B and $^{ 18}$C, the dipole strengths in the low energy region decline compared with those in $^{ 15}$B and $^{ 16}$C. We also discuss the energy weighted sum rule for the $E1$ transitions.
Categories: nucl-th