Image potential states of ultrathin NiO films: A time-resolved two-photon photoemission study

Well-ordered ultrathin films of NiO have been prepared on an Ag(001) substrate using molecular beam epitaxy. With the help of angle-resolved two-photon photoemission (2PPE) a series of image potential states (IPS) for film thicknesses of 2--4 monolayers (ML) has been identified. By time-resolved 2PPE, the lifetimes of the first three IPS and their dependence on the oxide film thickness have been determined. While the lifetimes of the (n=1) IPS are all in the range of 27--42 fs, the values for the (n=2) IPS decrease from 85 fs for 2 ML to 33 fs for 4 ML. These differences are discussed in terms of a coupling to the layer-dependent electronic structure of the NiO ultrathin films.