  Numerical calculations of surface and volume plasma excitations in silicon
and silicon-hydrogen nanoclusters in the range Si$_{10}$-Si$_{60}$ and
Si$_3$H$_8$-Si$_{39}$H$_{40}$ are performed. Some nanocluster structures were
obtained using the evolutionary algorithm, others were taken from the database.
The GW method was used to calculate the response function and self-energy of
the structures under study. The applied method shows the results consistent
with the experiment (except plasmaron artifacts) and sufficient sensitivity
allowing to investigate the effect of the cluster structure and size on the
specific properties of plasma excitations. In the studied silicon and
silicon-hydrogen nanoclusters the surface is one of the key factors affecting
the properties of the plasmons. Passivation of silicon dangling bonds on
cluster surface changes frequency of plasmons and significantly decreases their
damping. It makes the surface and volume plasmons to be clearly distinguishable
even in small clusters.
