The strength functions of isovector dipole resonances in the even oxygen isotopes $^{ 18\sim24}$O are calculated by using an extended version of the time-dependent Hartree-Fock theory known as the time-dependent density-matrix theory (TDDM). The results are compared with recent experimental data and also with a shell-model calculation. It is found that the observed isotope dependence of low-lying dipole strength is reproduced in TDDM when the strength of the residual interaction is properly chosen. It is also found that there is a difference between the TDDM prediction and the shell-model calculation for $^{ 24}$O.
Categories: nucl-th