  The dynamics of N/Z and mass equilibration are investigated in the reactions
112,124Sn + 239Pu by employing the isospin-dependent quantum molecular dynamics
model. It is found that N/Z and mass equilibration take place at different
collision stages. The N/Z relaxation is observed in the approaching phase (from
first contact to deepest contact) with a very short time, whereas interestingly
we find for the first time that mass equilibration only takes place in the
separation phase (from the deepest contact to re-separation), which are
explained by investigating the dynamical asymmetry between the approaching and
separation phases. The mass equilibration also could be clarified with a
dynamical potential energy surface. Our results provide a new insight into the
equilibration dynamics of the quantum systems.
