The nature of the interaction between highly oriented pyrolitic graphite (HOPG) and adsorbed Ti is examined by soft x-ray photoelectron spectroscopy. By monitoring the C $1s$ signal, it is found that Ti does not chemically bond to, but only interacts through the Van der Waals force with, a pristine HOPG surface. Time-dependent variations in the C $1s$ peak intensity are interpreted in terms of diffusion of adsorbed Ti atoms, which results in the formation of clusters. This behavior is further evidence for a weakly adsorbed overlayer and implies that this surface is a priori unable to support a layer-by-layer growth. A chemical reaction occurs only when oxygen is incorporated or defects are present. The presence of adatoms on HOPG has an effect on the screening process of the HOPG C $1s$ core hole, which can be detected even at the Van der Waals interaction level. This may be used as a measure of the strength of interfacial interactions, especially for cases of weak or small-scale interactions.

• Received 30 April 1998

DOI:https://doi.org/10.1103/PhysRevB.60.2827