  Exact solutions of Schroedinger and Pauli equations for charged particles in
an external stationary electromagnetic field of an arbitrary configuration are
constructed. Green functions of scalar and spinor particles are calculated in
this field. The corresponding equations for complex energy of particles bounded
by short range potential are deduced. Boundary condition typical for delta -
potential is not used in the treatment. Explicit analytical expressions are
given for the shift and width of a quasistationary level for different
configurations of the external field. The critical value of electric field in
which the idea of quasistationary level becomes meaningless is calculated. It
is shown that the common view on the stabilizing role of magnetic field
concerns only scalar particles.
