  We consider two--dimensional Yukawa theory in the scalar wave background
$\phi(t-x)$. If one takes as initial state in such a background the scalar
vacuum corresponding to $\phi=0$, then loop corrections to a certain part of
the Keldysh propagator, corresponding to the anomalous expectation value, grow
with time. That is a signal to the fact that under the kick of the $\phi(t-x)$
wave the scalar field rolls down the effective potential from the $\phi=0$
position to the proper ground state. We show the evidence supporting these
observations.
