Inelastic ion-surface collisions: scattering and dissociation of low energy benzene molecular cations

Abstract

We have studied collisions of benzene cations with a self-assembled monolayer surface of alkanethiol on gold substrate at kinetic energies ranging from 10 to 50 eV at 45° impact angle using a modified crossed-beam tandem mass spectrometer. Neutralization is the main reaction at all kinetic energies investigated. At ion kinetic energies below 10 eV, most reflected benzene cations are inelastically scattered with little fragmentation even though they lose most of their kinetic energy in the collision process. As ion energy is increased, the probability of surface-induced dissociation (SID) increases rather slowly. At all energies investigated neither elastic scattering nor SID occurs at the specular scattering angle. The total energy lost by benzene ions in inelastic/dissociative collisions increases as the ion kinetic energy is increased. Kinetic energy distributions of inelastically scattered benzene ions are rather broad and strongly dependent upon the ion kinetic energy. At lower energies, there is only one peak in the kinetic energy distribution of inelastically scattered benzene ions whereas at 50 eV benzene ions exhibit three peaks distinguishable in both energy and scattering angle. The high-energy peak corresponds to nearly zero energy loss (quasielastic scattering) and the lowest energy peak is quasithermalized. These two peaks are narrower than the central main peak suggesting a different dynamic mechanism for each of these collision processes.