  The Landau Zener method allows to measure very small tunnel splittings \Delta
in molecular clusters Fe_8. The observed oscillations of \Delta as a function
of the magnetic field applied along the hard anisotropy axis are explained in
terms of topological quantum interference of two tunnel paths of opposite
windings. Studies of the temperature dependence of the Landau Zener transition
rate P gives access to the topological quantum interference between exited spin
levels. The influence of nuclear spins is demonstrated by comparing P of the
standard Fe_8 sample with two isotopically substituted samples. The need of a
generalized Landau Zener transition rate theory is shown.
