  We report an Inelastic Neutron Scattering (INS) study of the fully deuterated
molecular compound K6[V15As6O42].9(D2O) (V15). Due to geometrical frustration,
the essential physics at low temperatures of the V15 cluster containing 15
coupled V4+ (S=1/2) is determined by three weakly coupled spin-1/2 on a
triangle. The INS spectra at low-energy allow us to directly determine the
effective exchange coupling J0 = 0.211 meV within the triangle and the gap 2
\Delta = 0.035(2)meV between the two spin-1/2 doublets of the ground state.
Results are discussed in terms of deviations from trigonal symmetry and
Dzyaloshinskii-Moriya (DM) interactions.
