Relativistic Effects and Gauge Invariance in Photon Absorption and Scattering

摘要

The interaction of light with matter is often described within the semi-classical framework where light is represented by an external time-dependent vector potential. We show that the transition probabilities so obtained suffer from two drawbacks: their semi-relativistic limit is delicate and they are not gauge invariant. We describe how a gauge-invariant transition probability can be obtained, for which the semi-relativistic limit is well defined. In this way, we obtain a new spin-position term which is not negligible in x-ray magnetic circular dichroism at the K-edge of transition metals.