To study isovector dipole responses of neutron-rich nuclei, we applied a time-dependent method of antisymmetrized molecular dynamics. The dipole resonances in Be, B, and C isotopes were investigated. In ${}^{10}\mathrm{Be}$, ${}^{15}\mathrm{B}$, and ${}^{16}\mathrm{C}$, 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 ${}^{16}\mathrm{C}$, we found that a remarkable peak at $E_x=14$ MeV corresponds to the coherent motion of four valence neutrons against a ${}^{12}\mathrm{C}$ core, whereas the GDR arises in the $E_x>20$ MeV region because of vibration within the core. In ${}^{17}\mathrm{B}$ and ${}^{18}\mathrm{C}$, the dipole strengths in the low-energy region decline compared with those in ${}^{15}\mathrm{B}$ and ${}^{16}\mathrm{C}$. We also discuss the energy-weighted sum rule for the $E1$ transitions.

• Received 22 July 2005

DOI:https://doi.org/10.1103/PhysRevC.72.064301