Quantum-Electrodynamical Theory of Atoms Interacting with High-Intensity Radiation Fields

Abstract

The interaction of a bound electron with external radiation fields of finite intensity is treated with the standard quantum-electrodynamical (QED) formalism of Feynman and Dyson. We first construct an equation for a bound electron in finite-intensity radiation fields, such as those encountered in early molecular-beam experiments and in recent masers and lasers. The Green's function for the equation so obtained enables us to compute the induced transition probabilities for various systems of interest. In this paper, we carry out the calculations on the two-level and three-level systems explicitly, where we find that, to order $e^4$, the QED method based on forward scattering and the semiclassical treatment differ. As we consider only the interaction of electrons with low-energy photons, we may ignore the virtual photon processes, in accordance with the low-energy theorem. As a result, this treatment contains only finite calculations. We demonstrate explicitly that our results are in qualitative agreement with the molecular-beam experiments of Kusch, for which the semiclassical treatment of Salwen fails to predict the results. Consequently, we expect an intensity-dependent effect which can be properly explained only by QED and not by the semiclassical treatment. We also include the effect of nonelectromagnetic relaxation on the induced transition probability of a two-level system, for which we obtain an expression slightly different from that used by Ramsey for the hydrogen maser. Finally, we derive some expressions which will be of interest in experiments related to lasers and masers.