  Fermion production in an external Coulomb field on de Sitter expanding
universe is studied. The amplitude and probability of pair production in an
external Coulomb field are computed and the cases of large/small values of the
expansion factor comparatively with the particle mass are studied. We obtain
from our calculations that the modulus of the momentum is no longer a conserved
quantity. We find that in the de Sitter space there are probabilities for
production processes where the helicity is no longer conserved. For a vanishing
expansion factor we recover the Minkowski limit where the amplitude of this
process vanishes. The rate of pair production in Coulomb field is found to be
important in the early universe when the expansion factor was large
comparatively with the particle mass.
