Frequency-dependent spin-lattice relaxation in 15N@C60

We report on pulsed electron-spin-resonance (ESR) investigations of endohedral 15N@C60. In order to study the spin-lattice relaxation and its underlying mechanisms, we performed saturation-recovery experiments both in X band (9.5 GHz) and W band (94 GHz). We find recovery signals with a non-exponential time behavior, which are due to several relaxation pathways. The temperature dependence of the relaxation rate can be understood by assuming that fluctuations of the fine structure and hyperfine interaction play the dominant role for the relaxation. These fluctuations are due to the damped quantum oscillator of the nitrogen atom inside C60. In our model, the observed differences in X band and W band are ascribed to an overdamped motion of this oscillator. (Author)