  We consider double ionization of atoms or ions by electron impact in the
presence of a static electric field. As in Wanniers analysis of the analogous
situation without external field the dynamics near threshold is dominated by a
saddle. With a field the saddle lies in a subspace of symmetrically escaping
electrons. Near threshold the classical cross section scales with excess energy
E like sigma=E^alpha, where the exponent alpha can be determined from the
stability of the saddle and does not depend on the field strength. For example,
if the remaining ion has charge Z=2, the exponent is 1.292, significantly
different from the 1.056 without the field.
