  Physical mechanisms of structural transformations in deposited metallic
clusters exposed to an electron beam of a transmission electron microscope
(TEM) are studied theoretically and computationally. Recent TEM experiments
with size-selected Au$_{923}$ clusters softly deposited on a carbon substrate
showed that the clusters undergo structural transformations from icosahedron to
decahedron and face-center cubic (fcc) structures upon exposure to a 200-keV
electron beam. In this paper, we demonstrate that the relaxation of collective
electronic (plasmon) excitations formed in deposited metal clusters can induce
the experimentally observed structural transformations. Such excitations in the
clusters are formed mainly due to the interaction with low-energy secondary
electrons emitted from a substrate. The characteristic occurrence times for
plasmon-induced energy relaxation events are several orders of magnitude
shorter than those for the momentum transfer events by energetic primary
electrons to atoms of the cluster. The theoretical analysis is complemented by
molecular dynamics simulations, which show that an icosahedral Au$_{923}$
cluster softly deposited on graphite is transformed into an fcc-like structure
due to the vibrational excitation of the cluster.
