  Tight-binding molecular dynamics simulations of photo-excitations in small Se
clusters (isolated Se$_8$ ring and helical Se chain) and glassy Se networks
(containing 162 atoms) were carried out in order to analyse the photo induced
instability inside the amorphous selenium. In the cluster systems after taking
an electron from the highest occupied molecular orbital to the lowest
unoccupied molecular orbital a bond breaking occurs. In the glassy networks
photoinduced volume expansion was observed and at the same time the number of
coordination defects changed significantly due to illumination.
