DETERMINATION OF ACTIVATION-ENERGIES FOR ION FRAGMENTATION BY SURFACE-INDUCED DISSOCIATION

We describe here a new method for extracting the activation energy for the formation of any fragment or reactively scattered ion that forms from a parent ion-surface collision. Our model is developed from first principles for collision-induced dissociation in the gas phase and th en modified for surface-induced dissociation (SID). This approach is c onceptually similar to that used for threshold collision-induced disso ciation measurements in that it assumes a similar functional form for the dissociation cross section, it takes into account the partitioning of energy between the projectile and the target, and it deconvolutes these over the kinetic energy distribution of the parent ion beam. The activation energy is extracted by an analysis of the energy-resolved mass spectra and the kinetic energy distribution spectra for the surfa ce-scattered ions. We test our method by determining the activation en ergies for the formation of the SiMe2+, SiMe+, SiD+ and Si+ fragment i ons from the d(9)-SiMe3+ parent ion scattered off a hexanethiolate sel f-assembled monolayer adsorbed on Au(111). The differences between the literature and SID activation energies are rationalized by considerat ion of the experimental uncertainty in the method.