Rabi oscillations as a tool to detect strong coupling of paramagnetic center with the nuclear spin bath

We show that strong coupling with the nuclear spin ensemble leaves an imprint on the nutation dynamics of the electron spin in the form of forced oscillations. The frequency of these oscillations equals Larmor precession frequency of the nuclear spins. This effect is evidenced by our experimental data on Rabi oscillations of paramagnetic nitroxyl radical TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) dispersed in glassy and crystalline ethanol. The data are interpreted in terms of a simple model that represents an impact of the proton spin bath as random local field with certain probability distribution. The electron spin coherence times of TEMPO in ethanol obtained by means of Hahn spin echo and Carr-Purcell-Meiboom-Gill sequences are compared with the calculations based on a spin diffusion model.