  The low frequency excitations of C60 chains inserted inside single-walled
carbon nanotubes (SWNTs) have been studied by inelastic neutron scattering
(INS) on a high quality sample of peapods. The comparison of the
neutron-derived generalized phonon density of states (GDOS) of the peapods
sample with that of a raw SWNTs allows the vibrational properties of the C60
chains encapsulated in the hollow core of the SWNTs to be probed. Lattice
dynamical models are used to calculate the GDOS of chains of monomers, dimers
and polymers inserted into SWNTs, which are compared to the experimental data.
The presence of strong interactions between C60 cages inside the nanotube is
clearly demonstrated by an excess of mode density in the frequency range around
10 meV. However, the presence of a quasi-elastic signal indicates that some of
the C60\'s undergo rotational motion. This suggests that peapods are made from
a mixture of C60 monomers and C60 n-mer (dimer, trimer ... polymer) structures.
