Ground state tunneling due to a distribution of tunnel splittings in Mn ₁₂-acetate

We report on detailed measurements of the magnetization of a single crystal of Mn ₁₂-acetate in a swept magnetic field for a set of closely spaced temperatures. We show that under some circumstances relaxation that should proceed from the ground state appears to be missing under conditions where one would expect it to be present. We argue that this enigma implies there is a distribution of tunnel splittings, so that the (normalized) magnetization determines the fraction of distributed molecules that tunnel instead of the tunneling probability of an identical set of molecules.