Proton-neutron structure of the effective quadrupole-octupole coupled E1 transition operator

Electric dipole transitions between 3₁^- octupole-phonon states, one-phonon 2_1,ms⁺ mixed-symmetry states of ⁹²Zr, ⁹⁴Mo, ⁹⁶Ru, and ¹⁴²Ce are observed, provide the first evidence for F-vector E1 transitions in nuclei. For effective boson models of the nuclear valence shell, F spin is the analog of isospin in the shell model. The 3₁ ^-→ 2_1,ms⁺ E1 transitions are stronger than those of the 2₁⁺ state. The effective E1 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 operator in microscopic models.