Binary-reaction spectroscopy of ^99,100Mo: Intruder alignment systematics in N=57 and N=58 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 I^π=11/2⁻ isomeric state in ⁹⁹Mo has been extended through the first alignment up to a tentative spin∕parity of (43/2⁻). The results are compared with self-consistent, cranked-mean-field calculations using a Woods-Saxon potential. The alignment systematics of the intruder h_11∕2 bands in the N=57 isotones from Mo (Z=42) to Cd (Z=48) and the yrast sequences in their N=58 even-even neighbors are discussed. An overall picture emerges, where the alignment properties evolve from being due to positive-parity neutrons in the ₄₈¹⁰⁵Cd to predominantly (g_9∕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 g_9∕2 shell with decreasing proton number. These data provide excellent examples of rotational-alignment phenomena in weakly deformed nuclei.