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In this paper we study the structure of even-even Ba isotopes with neutron number N=84--90 within the framework of nucleon-pair approximation of the shell model. We use a phenomenological Hamiltonian, which includes single-particle energies, monopole, quadrupole, and octupole pairing energies, as well as quadrupole-quadrupole and octupole-octupole interactions. Our calculated energy levels and B (E2) values are in good agreement with experimental data. The alternating parity doublets are well reproduced by considering F^- nucleon pairs with spin-three and negative parity, which demonstrates the importance of F^- pair in octupole deformation. The octupole-octupole interaction is found to favor stronger B (E3) transitions and suppress B (E2) transitions in the low-lying states of nuclei in these even-even Ba isotopes.
Yin et al. (Fri,) studied this question.