  We consider the scattering of charged particles on particular electromagnetic
fields which have properties analogous to gravitational horizons. Classically,
particles become causally excluded from regions of spacetime beyond a null
surface which we identify as the `electromagnetic horizon'. In the quantum
theory there is pair production at the horizon via the Schwinger effect, but
only one particle from the pair escapes the field. Furthermore, unitarity
appears to be violated when crossing the horizon, and there is no well-defined
S-matrix. Despite this, we show how to use the perturbiner method to construct
`amplitudes' which contain all the dynamical information required to construct
observables related to pair creation, and to radiation from particles
scattering on the background.
