  We investigate time-dependent probability for a Brownian particle passing
over the barrier to stay at a metastable potential pocket against escaping over
the barrier. This is related to whole fusion-fission dynamical process and can
be called the reverse Kramers problem. By the passing probability over the
saddle point of inverse harmonic potential multiplying the exponential decay
factor of a particle in the metastable potential, we present an approximate
expression for the modified passing probability over the barrier, in which the
effect of reflection boundary of potential is taken into account. Our
analytical result and Langevin Monte-Carlo simulation show that the probability
passing and against escaping over the barrier is a non-monotonous function of
time and its maximal value is less than the stationary result of passing
probability over the saddle point of inverse harmonic potential.
