Echo trains in pulsed electron spin resonance of a strongly coupled spin ensemble.

We report on a phenomenon in electron spin resonance experiments of phosphorus donors. We observe multiple echo signatures (echo trains) after a conventional Hahn echo pulse sequence if the spin system is strongly coupled to the resonator due to the coherent exchange of excitations between both subsystems. We can quantitatively explain the decay of the echo train by simulations based on a rate equation model. This model allows us to extract the spin coherence time $T_2$ of the spin ensemble in a single-shot pulsed spin resonance experiment.