We have studied electric dipole $\mathrm{}\left(E1\mathrm{}\right)\mathrm{}$ strength distribution in neutron and proton drip line nuclei ${}_{\hspace{0.5em}4}^{12}{\mathrm{Be}}_8,$ ${}_{\hspace{0.5em}4}^{14}{\mathrm{Be}}_{10},$ and ${}_{\hspace{0.5em}8}^{13}{\mathrm{O}}_5$ by large scale shell model calculations with the effect of loosely-bound single particle wave functions. Large $E1\mathrm{}$ strength is found in the low excitation energy region below $E_x=3\mathrm{}\hspace{0.5em}\mathrm{MeV}$ in both ${}_{\hspace{0.5em}4}^{12}{\mathrm{Be}}_8$ and ${}_{\hspace{0.5em}8}^{13}{\mathrm{O}}_5,$ while strong $E1\mathrm{}$ strength is found only in high excitation energy region above $E_x=7\mathrm{}\hspace{0.5em}\mathrm{MeV}$ in ${}_{\hspace{0.5em}4}^{14}{\mathrm{Be}}_{10}.$ The effect of extended wave functions and the coherence in the transition amplitudes enhance significantly the $E1\mathrm{}$ strength in the low energy region. A manifestation of melting of the shell magicity at $N=8\mathrm{}$ and $Z=8\mathrm{}$ is pointed out in the systematics of $E1\mathrm{}$ transitions in Be and O isotopes.

• Received 21 August 2000

DOI:https://doi.org/10.1103/PhysRevC.63.034310