Observation of Electron Spin Dynamics on Hundreds-of-Microseconds Timescales in High-Field Magic Angle Spinning EPR

Dynamic nuclear polarization (DNP) efficiency critically depends on electron-nuclear polarization transfer, necessitating a thorough understanding of electron spin dynamics under DNP-relevant conditions, namely, high magnetic fields and magic angle spinning (MAS). While high-field electron paramagnetic resonance (EPR) under MAS is the only tool capable of probing these dynamics, its application has been limited by technological constraints and methodological gaps, leaving the microscopic behavior of electron spin packets under MAS largely unexplored experimentally. Here we demonstrate the first rotor-synchronized three-pulse stimulated echo MAS-EPR experiments at 6.9 T with spinning frequencies up to 37 kHz. This approach enables the characterization of electron spin dynamics on timescales of hundreds of microseconds under MAS, revealing its strong dependence on both EPR line anisotropy and MAS rate. We further validated our experimental findings with time-domain numerical simulations. This work establishes a robust methodology for investigating electron spin dynamics in MAS-DNP on the experimentally relevant timescales.