Quantum electrodynamic approach to multiphoton ionization in strong fields

In this paper we consider multiphoton ionization (MPI) within the framework of quantum electrodynamics (QED). Based on the quantized-field version of the Volkov solution, a formal scattering theory is developed. In particular, we include the non-laser radiation modes in our, Hamiltonian, to take into account the contribution from the spontaneous emission process to the transition amplitude. It is shown that, treating the radiation field with QED theory, one may firstly obtain the above threshold ionization energy distribution spectrum in an analytical form. Moreover, in addition to the well known semiclassical theory, the framework presented here; derived from a QED perspective, provides a new picture of the MPI process.