  Radiative and non-radiative electron spin flip probabilities are analysed in
both plane wave and focussed laser backgrounds. We provide a simple and
physically transparent description of spin dynamics in plane waves, and
demonstrate that there exists a kinematic regime in which the usual leading
order perturbative hierarchy of QED is reversed, and non-radiative loop effects
dominate over radiative tree-level spin-flips. We show that while this
loop-dominance becomes suppressed in focussed laser pulses due to a high
sensitivity to field geometry, there is nevertheless a regime in which, in
principle, loop effects on spin transitions can be discerned.
