Energetic ion-stimulated desorption of physisorbed molecules

We have conducted an experimental investigation into molecular desorption stimulated by 8 keV Ar⁺ ions. The investigated systems are comprised of aromatic molecules (benzene and phenol) adsorbed to an Ag(111) surface. Resonance-enhanced laser ionization coupled with time-of-flight mass spectrometry provide the ability to obtain quantum state resolved kinetic energy distributions of the desorbed molecules. Our results indicate that the desorption mechanisms for the molecules are dictated by the molecular coverage and determine if the molecules are desorbed in an internally excited state. We use specific mechanisms observed in molecular dynamics simulations reported in the literature to describe our experimental observations. In the low coverage regime, ballistic collisions between dislocated silver substrate atoms and the adsorbed molecules lead to the emission of energetic molecules. A collision between a single substrate atom and an adsorbed molecule leads to the emission of translationally and internally hotter molecules. A collision between an adsorbed molecule and several substrate atoms with similar momentum leads to the emission of slower, internally cooler molecules. In multilayer systems, a gentler mechanism, such as a molecular collision cascade or localized heating, generates the emission of translationally slow molecules. Temperature-programmed desorption studies allow characterization of the benzene film structure and show how the emission characteristics of the desorbed molecules change concomitantly with changes in the film structure. In general, we provide a framework describing the collision events responsible for stimulated molecular desorption.