  Inelastic neutron scattering (INS) experiments have been performed on the
Cu$_3$ triangular molecular nanomagnet using powder samples. In the medium
resolution INS spectrum measured, there are two peaks at $\hbar\omega=0.5$ and
0.6 meV. Comparing the observed $Q$ dependences of these peaks with
calculations, these two INS peaks originate from the Cu$_3$ cluster. From the
observed peak position, width, and intensity, we have determined the optimum
parameters of the spin Hamiltonian consisted of the antiferromagnetic exchange
and the Dzyaloshinsky-Moriya interactions, which can also reproduce the
magnetic susceptibility measurement. In addition, we have directly observed
that the ground state quartet is split into two doublets with the energy
separation of 0.103 meV using high-resolution neutron spectroscopy, which
exactly corresponds to that expected from the optimum parameters obtained from
the medium resolution xperiment. The temperature dependences of the integrated
intensities of the 0.5 and 0.6 meV peaks are well reproduced by the Boltzmann
distribution of the energy levels of the model Hamiltonian below 10 K.
Furthermore, the inelastic peaks were visible even at very high temperatures as
50 K. This indicates extraordinary weak coupling between phonons (or any other
perturbations) and spin states in the Cu$_3$ cluster, compared to the other
known molecular nanomagnets.
