Direct measurement of the tunneling rate of the magnetization in Fe8 via 57Fe nuclear spin-lattice relaxation by strong collision

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⁵⁷Fe and ¹H relaxation measurements have been performed in single crystal and oriented powder of enriched ⁵⁷Fe8 molecular cluster in the temperature range 0.05--1.7 K in zero external field and with small perturbing longitudinal field (< 1 T). On the basis of the experimental results it is argued that in zero external field the nuclear spin-lattice relaxation (1/T₁) mechanism is driven by a strong collision mechanism whereby 1/T₁ is a direct measure of the incoherent tunneling probability in the low lying magnetic energy states of the molecular nanomagnet. The approximate value of the effective tunneling rate vs T and H derived directly from 1/T₁ is shown to be consistent with theoretical estimates based on known parameters of the Hamiltonian.