First experimental evidence for two-phonon octupole-γ-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 ${\mathit{J}}^{\mathrm{\pi }}$=$1^{\mathrm{-}}$ states are interpreted as two-phonon excitations due to the coupling of octupole (J=$3^{\mathrm{-}}$,K=1) and quadrupole γ 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.