  The adsorption of ethylene molecule on neutral, anionic and cationic gold
clusters consisting of up to 10 atoms has been investigated using
density-functional theory. It is demonstrated that C2H4 can be adsorbed on
small gold clusters in two different configurations, corresponding to the pi-
and di-sigma-bonded species. Adsorption in the pi-bonded mode dominates over
the di-sigma mode over all considered cluster sizes n, with the exception of
the neutral C2H4-Au5 system. A striking difference is found in the
size-dependence of the adsorption energy of C2H4 bonded to the neutral gold
clusters in the pi and di-sigma configurations. The important role of the
electronic shell effects in the di-sigma mode of ethylene adsorption on neutral
gold clusters is demonstrated. It is shown that the interaction of C2H4 with
small gold clusters strongly depends on their charge. The typical shift in the
vibrational frequencies of C2H4 adsorbed in the pi- and the di-sigma
configurations gives a guidance to experimentally distinguish between the two
modes of adsorption.
