  We present a detailed study of a monolayer film of
3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on Ag(111) (the
PTCDA/Ag(111) system). The study is done within density functional theory with
the use of the periodic slab model. The slab is chosen to contain a PTCDA
monolayer film on a silver thin film of different thicknesses (6, 9, and 12
layers) with the (111) orientation. We show that one of two surface states of
the pure Ag(111) films transforms into an unoccupied interface state due to the
adsorbate-substrate interaction. The relation of the resulting state to the
unoccupied state that has been experimentally observed in the PTCDA/Ag(111)
system by scanning tunneling and two photon photoemission spectroscopy is
discussed.
