  We show how to use the worldline-instanton formalism to calculate the
momentum spectrum of the electron-positron pairs produced by an electric field
that depends on both space and time. Using the LSZ reduction formula with a
worldline representation for the propagator in a spacetime field, we make use
of the saddle-point method to obtain a semiclassical approximation of the
pair-production spectrum. In order to check the final result, we integrate the
spectrum and compare with the results obtained using a previous instanton
method for the imaginary part of the effective action.
