Contrasting behaviour of proton h 9 2 and h 11/NU> 2 bands in 175,177,179,181Ir interpreted in an intruder model

Rotational bands in the odd-proton isotopes Ir, Ir, Ir and Ir based on the h, h, i and d orbitals have been identified, the first three to high spin. In all cases the h band shows backbending with a crossing frequency and alignment gain consistent with that expected from the neutron i alignment. In contrast the h bands (and the fragments of d bands observed) show complex alignment curves. These alignments can be explained using a phenomenological three-band model which incorporates a deformed intruder band, analogous to that proposed in the very light Pt and Os nuclei. Mixed wave functions from the model reproduce the observed B(M1)/B(E2) ratios. The presence of the intruder band in the h and d configurations and its absence in the h band is compatible with its proposed association with an (h ) pair excitation