  The properties of Shockley-type interface states between $\pi$-conjugated
organic molecular layers and metal surfaces are investigated by time-resolved
two-photon photoemission experiments and density functional theory. For
perylene- and naphthalene-tetracarboxylic acid dianhydride (PTCDA and NTCDA)
adsorbed on Ag(111), a common mechanism of formation of the interface state
from the partly occupied surface state of the bare Ag(111) is revealed. The
energy position is found to be strongly dependent on the distance of the
molecular carbon rings from the metal and their surface density. Bending of the
carboxyl groups enhances the molecular overlap of the interface state.
