Proton NMR for Measuring Quantum Level Crossing in the Magnetic Molecular Ring Fe10

The proton nuclear spin-lattice relaxation rate 1/T{sub 1} has been measured as a function of temperature and magnetic field (up to 15thinspthinspT) in the molecular magnetic ring Fe{sub 10}( OCH{sub 3}){sub 20}(O{sub 2}CCH{sub 2} Cl){sub 10} (Fe10). Striking enhancement of 1/T{sub 1} is observed around magnetic field values corresponding to a crossing between the ground state and the excited states of the molecule. We propose that this is due to a cross-relaxation effect between the nuclear Zeeman reservoir and the reservoir of the Zeeman levels of the molecule. This effect provides a powerful tool to investigate quantum dynamical phenomena at level crossing. {copyright} {ital 1999} {ital The American Physical Society}