  We present low temperature magnetometry measurements on a new Mn3
single-molecule magnet (SMM) in which the quantum tunneling of magnetization
(QTM) displays clear evidence for quantum mechanical selection rules. A QTM
resonance appearing only at elevated temperatures demonstrates tunneling
between excited states with spin projections differing by a multiple of three:
this is dictated by the C3 symmetry of the molecule, which forbids pure
tunneling from the lowest metastable state. Resonances forbidden by the
molecular symmetry are explained by correctly orienting the Jahn-Teller axes of
the individual manganese ions, and by including transverse dipolar fields.
These factors are likely to be important for QTM in all SMMs.
