Observation of low-lying collective dipole transitions in the rare-earth nuclei 172,174,176Yb

Precise experimental data have been obtained on the distribution of low lying collective spin-one states in the rare-earth nuclei 172,174,176Yb using high-resolution nuclear resonance fluorescence. We report on excitation energies, spins and decay-widths of these states. The data show that the dipole strength below 4 MeV is strongly fragmented. We discuss the influence of the fragmentation on the so-called M1 'scissors mode', and compare upper limits for M1 strengths to those measured for the neighbouring nuclei. (orig.)