  Scattering and electron-positron pair production by a one-dimensional
potential is considered in the framework of the $S-$matrix formalism. The
solutions of the Dirac equation are classified according to frequency sign. The
Bogoliubov transformation relating the in- and out-states are given. We show
that the norm of a solution of the wave equation is determined by the largest
amplitude of its asymptotic form when $x_3\to \pm\infty$. For a number of
potentials we give the explicit expressions for the complete in- and out-sets
of orthonormalized wave functions. We note that in principle virtual vacuum
processes in external field influence the phase of wave function of scattered
particle..
