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Powder and single-crystal samples of the dimer compound Cs3Cr28r9 have been studied by inelastic neutron scattering. At 1.6 K, the singlet-to-triplet dimer excitation exhibits pronounced energy dispersion as a result of weak interdimer interactions. The results are interpreted within the random-phase approximation with use of a Heisenberg Hamiltonian and considering only nearestneighbor interactions. The agreement between theory and experiment is excellent. Three exchange parameters have been obtained at 1.6 K: J = -1.03 meV (intradimer), J~= -0.054 meV (interdimer, intrasublattice), and J, = -0.039 meV (interdimer, intersublattice). The excitation shows softmode behavior. However, an ordering does not occur above 1.6 K.

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