Phonon-assisted tunneling in the quantum regime of Mn-12 acetate

Longitudinal or transverse magnetic fields applied on a crystal of Mn-12 ac etate allows one to observe independent tunnel transitions between m = -S p and m = S - n - p (n = 6-10, p = 0-2 in longitudinal field and n = p = 0 in transverse field). We observe a smooth transition tin longitudinal) fro m coherent ground-state to thermally activated tunneling. Furthermore, two ground;state relaxation regimes show a crossover between quantum spin relax ation far from equilibrium and near equilibrium, when the environment destr oys multimolecule correlations. Finally, we stress that the complete Hamilt onian of Mn-12 should contain odd spin operators of low order.