Dipole excitations in the semimagic $N=50\mathrm{}$ nucleus ${}^{87}\mathrm{Rb}$ were investigated at the Stuttgart Dynamitron facility using bremsstrahlung with an end-point energy of 4.0 MeV. The widths $\Gamma$ or the reduced excitation probabilities $B\left(\Pi 1\right)\uparrow$ of 18 states were determined for the first time. The magnetic dipole excitations are well reproduced in the framework of the shell model, however, these calculations cannot describe the observed electric dipole excitations. The ${1/2}^{+}$ state at 3060 keV is proposed to be the weak coupling of an $f_{5/2}$ proton hole to the $3^{-}$ octupole vibrational state in the $N=50\mathrm{}$ core ${}^{88}\mathrm{Sr}.$ The relatively strong $E1\mathrm{}$ transition from that state to the ground state is explained as mainly the neutron $h_{11/2}\to g_{9/2}$ transition. The breakup of the $N=50\mathrm{}$ core and neutron excitations into the $h_{11/2}$ shell are essential to describe electric dipole excitations, but neutron-core excitations do not play an important role for the structure of magnetic dipole excitations.

• Received 30 November 2001

DOI:https://doi.org/10.1103/PhysRevC.65.054315