  The $\alpha $-particle production in the dissociation of $^{9}$Be on
$^{209}$Bi and $^{64}$Zn at energies just above the Coulomb barrier is studied
within the two-center shell model approach. The dissociation of $^{9}$Be on
$^{209}$Bi is caused by a molecular single-particle effect (Landau-Zener
mechanism) before the nuclei reach the Coulomb barrier. Molecular
single-particle effects do not occur at that stage of the collision for
$^{9}$Be+$^{64}$Zn, and this explains the absence of fusion suppression
observed for this system. The polarisation of the energy level of the last
neutron of $^{9}$Be and, therefore the existence of avoided crossings with that
level, depends on the structure of the target.
