  We have performed classical molecular dynamics simulation of $Ar_{13}$
cluster to study the behavior of collective excitations. In the solid ``phase''
of the cluster, the collective oscillation of the monopole mode can be well
fitted to a damped harmonic oscillator. The parameters of the equivalent damped
harmonic oscillator-- the damping coefficient, spring constant, time period of
oscillation and the mass of the oscillator -- all show a sharp change in
behavior at a kinetic temperature of about $7.0^oK$. This marks yet another
characteristic temperature of the system, a temperature $T_s$ below which
collective excitations are very stable, and at higher temperatures the single
particle excitations cause the damping of the collective oscillations. We argue
that so long as the cluster remains confined within the global potential energy
minimum the collective excitations do not decay; and once the cluster comes out
of this well, the local potential energy minima pockets act as single particle
excitation channels in destroying the collective motion. The effect is manifest
in almost all the physical observables of the cluster.
