  We will study the Klein-Gordon's field with an homogeneous external
potential, which does not depend on $\h$. We will construct the Fock's space
corresponding to our problem and we will see that there are phenomena of
creation and anihilation of pairs particle-antiparticle. Finally, we will see
that in dimension 1, when $\h\to 0$, these phenomena disappear. However, in
dimension 2 or 3, when $\h\to 0$, the creation probability of
particle-antiparticle pairs is not zero.
