Electron spin dynamics during microwave pulses studied by 94 GHz chirp and phase-modulated EPR experiments

Electron spin dynamics during microwave irradiation are of increasing interest in electron paramagnetic resonance (EPR) spectroscopy, as locking electron spins into a dressed state finds applications in EPR and dynamic nuclear polarization (DNP) experiments. Here, we show that these dynamics can be probed by modern pulsed EPR experiments that use arbitrary waveform generators to produce shaped microwave pulses. We employ phase-modulated pulses to measure Rabi nutations, echoes, and echo decays during spin locking of a BDPA (1,3-bisdiphenylene-2-phenylallyl) radical at 94 GHz EPR frequency. Depending on the initial state of magnetization, different types of echoes are observed. We analyze these distinct coherence transfer pathways and measure the decoherence time $T_{2\rho }$ , which is a factor of 2-3 times longer than $T_m$ . Furthermore, we use chirped Fourier transform EPR to detect the evolution of magnetization profiles. Our experimental results are well reproduced using a simple density matrix model that accounts for $T_{2\rho }$ relaxation in the spin lock (tilted) frame. The results provide a starting point for optimizing EPR experiments based on hole burning, such as electron-nuclear double resonance or ELectron-electron DOuble Resonance (ELDOR)-detected NMR.