  Below 360 mK, Fe magnetic molecular clusters are in the pure quantum
relaxation regime and we show that the predicted square-root time relaxation is
obeyed, allowing us to develop a new method for watching the evolution of the
distribution of molecular spin states in the sample. We measure as a function
of applied field H the statistical distribution P(\xi_H) of magnetic energy
bias \xi_H$ acting on the molecules. Tunneling initially causes rapid
transitions of molecules, thereby digging a hole in P(\xi_H) (around the
resonant condition \xi_H = 0). For small initial magnetization values, the hole
width shows an intrinsic broadening which may be due to nuclear spins.
