Electric dipole transitions between $3_1^{-}$ octupole-phonon states and one-phonon $2_{1,\mathrm{ms}}^{+}$ mixed-symmetry states of ${}^{92}\mathrm{Zr},$ ${}^{94}\mathrm{Mo},$ ${}^{96}\mathrm{Ru},$ and ${}^{142}\mathrm{Ce}$ are observed and provide the first evidence for F-vector $E1\mathrm{}$ transitions in nuclei. For effective boson models of the nuclear valence shell, F spin is the analog of isospin in the shell model. The $3_1^{-}\to 2_{1,\mathrm{ms}}^{+}$ $E1\mathrm{}$ transitions are stronger than those of the $2_1^{+}$ state. The effective $E1\mathrm{}$ operator in the sdf-interacting boson model-2 is concluded to have F-vector character, in analogy with the well-known isovector character of the $E1\mathrm{}$ operator in microscopic models.

• Received 21 November 2002

DOI:https://doi.org/10.1103/PhysRevC.68.031305