Binary-reaction spectroscopy of 99,100 Mo: Intruder alignment systematics in N557 and N558 isotones

The near-yrast states of 99,100Mo have been studied following their population via a binary reaction between a 136Xe beam and a thin, self-supporting 100Mo target. The yrast sequence in 100Mo has been extended to a tentative spin/parity (20+), while the decoupled band built on the Iπ=11/2- isomeric state in 99Mo has been extended through the first alignment up to a tentative spin/parity of (45/2-). The results are compared with self-consistent, cranked-mean-field calculations using a Woods-Saxon potential. The alignment systematics of the intruder h11/2, bands in the N=51 isotones from Mo (Z=42) to Cd (Z=48) and the yrast sequences in their W=58 even-even neighbors are discussed. An overall picture emerges, where the alignment properties evolve from being due to positive-parity neutrons in the 48 105Cd to predominantly (g9/2)2 proton crossings closer to the Z=40 subshell. Qualitatively, this can be explained by an increase in the quadrupole deformation and a simultaneous lowering of the proton Fermi surface in the g9/2 shell with decreasing proton number. These data provide excellent examples of rotational-alignment phenomena in weakly deformed nuclei.