Evidence for octupole collectivity in ${}^{172}$Pt

Excited states in the extremely neutron-deficient nucleus ${}^{172}$Pt were populated via ${}^{96}$Ru(${}^{78}$Kr,2p) and ${}^{92}$Mo(${}^{83}$Kr,3n) reactions. The level scheme has been extended up to an excitation energy of ≈5 MeV and tentative spin-parity assignments up to I${}^{\pi }$=18${}^{+}$. Linear polarization and angular distribution measurements were used to determine the electromagnetic E1 character of the dipole transitions connecting the positive-parity ground-state band with an excited side-band, firmly establishing it as a negative-parity band. The lowest member of this negative-parity structure was firmly assigned spin-parity 3${}^{-}$. In addition, we observed an E3 transition from this 3${}^{-}$ state to the ground state, providing direct evidence for octupole collectivity in ${}^{172}$Pt. Large-scale shell model (LSSM) and total Routhian surface (TRS) calculations have been performed, supporting the interpretation of the 3${}^{-}$ state as a collective octupole-vibrational state.