FIRST EXPERIMENTAL EVIDENCE FOR TWO-PHONON OCTUPOLE-GAMMA -VIBRATIONAL EXCITATIONS IN DEFORMED NUCLEI

Systematic nuclear resonance fluorescence experiments, including model independent parity determinations, provided evidence for enhanced E1 excitations near 2.5 MeV excitation energy in the deformed nuclei $^{150}\mathrm{Nd}$, $^{160}\mathrm{Gd}$, and $^{162,164}\mathrm{Dy}$. The corresponding ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${1}^{\mathrm{\ensuremath{-}}}$ states are interpreted as two-phonon excitations due to the coupling of octupole (J=${3}^{\mathrm{\ensuremath{-}}}$,K=1) and quadrupole \ensuremath{\gamma} vibrations (J=${2}^{+}$,K=2). The results can be explained in the framework of the sdf interacting-boson-approximation model and the dynamic collective model describing octupole vibrations and their coupling to quadrupole vibrations in deformed nuclei.