The near-yrast states of ${}^{99,100}\mathrm{Mo}$ have been studied following their population via a binary reaction between a ${}^{136}\mathrm{Xe}$ beam and a thin, self-supporting ${}^{100}\mathrm{Mo}$ target. The yrast sequence in ${}^{100}\mathrm{Mo}$ has been extended to a tentative spin∕parity $\left({20}^{+}\right)$, while the decoupled band built on the $I^{\pi }={\frac{11}{2}}^{-}$ isomeric state in ${}^{99}\mathrm{Mo}$ has been extended through the first alignment up to a tentative spin∕parity of $\left({\frac{43}{2}}^{-}\right)$. 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 $\left(Z=42\right)$ to Cd $\left(Z=48\right)$ 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 ${}_{48}{}^{105}\mathrm{Cd}$ to predominantly ${\left(g_{9∕2}\right)}^2$ 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.

• Received 22 July 2003

DOI:https://doi.org/10.1103/PhysRevC.68.044313