Ionization and fragmentation of polycyclic aromatic hydrocarbon clusters in collisions with keV ions

Abstract

We report on an experimental study of the ionization and fragmentation of clusters of $k$ polycyclic aromatic hydrocarbon (PAH) molecules using anthracene, C${}_{14}$H${}_{10}$, or coronene, C${}_{24}$H${}_{12}$. These PAH clusters are moderately charged and strongly heated in small impact parameter collisions with 22.5-keV He${}^{2+}$ ions, after which they mostly decay in long monomer evaporation sequences with singly charged and comparatively cold monomers as dominating end products. We describe a simple cluster evaporation model and estimate the number of PAH molecules in the clusters that have to be hit by He${}^{2+}$ projectiles for such complete cluster evaporations to occur. Highly charged and initially cold clusters are efficiently formed in collisions with 360-keV Xe${}^{20+}$ ions, leading to cluster Coulomb explosions and several hot charged fragments, which again predominantly yield singly charged, but much hotter, monomer ions than the He${}^{2+}$ collisions. We present a simple formula, based on density-functional-theory calculations, for the ionization energy sequences as functions of coronene cluster size, rationalized in terms of the classic electrostatic expression for the ionization of a charged conducting object. Our analysis indicates that multiple electron removal by highly charged ions from a cluster of PAH molecules rapidly may become more important than single ionization as the cluster size $k$ increases and that this is the main reason for the unexpectedly strong heating in these types of collisions.