  We report on inelastic neutron scattering (INS) measurements on the molecular
spin ring CsFe$_8$, in which eight spin-5/2 Fe(III) ions are coupled by
nearest-neighbor antiferromagnetic Heisenberg interaction. We have recorded INS
data on a non-deuterated powder sample up to high energies at the
time-of-flight spectrometers FOCUS at PSI and MARI at ISIS, which clearly show
the excitation of spin waves in the ring. Due to the small number of spin
sites, the spin-wave dispersion relation is not continuous but quantized.
Furthermore, the system exhibits a gap between the ground state and the first
excited state. We have modeled our data using exact diagonalization of a
Heisenberg-exchange Hamiltonian together with a small single-ion anisotropy
term. Due to the molecule's symmetry, only two parameters $J$ and $D$ are
needed to obtain excellent agreement with the data. The results can be well
described within the framework of the rotational-band model as well as
antiferromagnetic spin-wave theories.
