Non-yrast nuclear spectra in a model of coherent quadrupole-octupole motion

A model assuming coherent quadrupole-octupole vibrations and rotations is applied to describe non-yrast energy sequences with alternating parity in several even-even nuclei from different regions, namely, 152,154Sm, 154,156,158Gd, ²³⁶U, and ¹⁰⁰Mo. Within the model scheme, the yrast alternating-parity band is composed by the members of the ground-state band and the lowest negative-parity levels with odd angular momenta. The non-yrast alternating-parity sequences unite levels of β bands with higher negative-parity levels. The model description reproduces the structure of the considered alternating-parity spectra together with the observed B($E1$), B($E2$), and B($E3$) transition probabilities within and between the different level sequences. B($E1$) and B($E3$) reduced probabilities for transitions connecting states with opposite parity in the non-yrast alternating-parity bands are predicted. The implemented study outlines the limits of the considered band-coupling scheme and provides estimations about the collective energy potential, which governs the quadrupole-octupole properties of the considered nuclei.