Scattering of photons by an external current

Considering the interaction of a quantized electromagnetic field with a prescribed external c-number current, we show that, for each current, there is a representation of the field operators in a separable Hilbert space and a unitary S operator transforming the free incoming field into the free outgoing one. The representation, a generalized product representation, depends on the current so that, in general, for two different currents the corresponding representations are unitarily inequivalent. Photon states and asymptotic observables are defined for the case of an infrared divergent current.