The nature of the observed free-electron-like state in a PTCDA monolayer on Ag(111)

A free-electron-like band has recently been observed in a monolayer of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) molecules on Ag(111) by two-photon photoemission (Schwalb et al 2008 Phys. Rev. Lett. 101 146801) and scanning tunneling spectroscopy (Temirov et al 2006 Nature 444 350). Using density functional theory calculations, we find that the observed free-electron-like band originates from the Shockley surface state band being dramatically shifted up in energy by the interaction with the adsorbed molecules, while it also acquires a substantial admixture with a molecular band