Collisions of Silylium Cations with Hydroxyl-Terminated and other Self-Assembled Monolayer Surfaces: Reactions, Dissociation and Surface Characterization

Silylium cations, SiCl3+ and Si(CH3)(3)(+), undergo dissociative ion/surface reactions in the course of low-energy (20-90 eV) collisions with hydroxyl-terminated (HO-SAM), hydrocarbon (H-SAM), and fluorocarbon (F-SAM) self-assembled monolayer surfaces. Formation of the substitution product, SiCl2F+, upon collision of SiCl3+ With the F-SAM surface is the result of a transhalogenation reaction. In an analogous fashion, one observes substitution of a chlorine in the SiCl3+ projectile ion by either an OH group from the HO-SAM surface or a CH3 group from the H-SAM surface to form the scattered reaction products, SiCl2OH+ and SiCl2CH3+, respectively. The concomitant transfer of a Cl atom from the projectile ion into the surface is indicated by the sputtered ion, CH2Cl+. The scattered product SiCl(OH)(2)(+) involves disubstitution, and reaction with more than one chain at the surface. These and related reactions involve the activation of C-O, C-F, C-C, C-H, and O-H bonds at the appropriate surface, and they occur after, or in concert with, surface-induced dissociation of the polyatomic projectile. Surface effects on the dissociation of projectile ions are studied using the Si(C2H5)(4)(.+) ion, and threshold values for translational to internal energy (T double right arrow V) conversion for this ion are measured as 13%, 13%, and 20% for the H-SAM, MO-SAM, and F-SAM surfaces, respectively. At higher collision energies, (>40 eV), the MO-SAM surface demonstrated greater internal energy conversion efficiency than the H-SAM surface. The process of neutralization and the accompanying release of chemically sputtered ions also served to distinguish the three surfaces. Decreased neutralization at the F-SAM surface is associated with increased amounts of dissociatively and reactively scattered product ions. Thermodynamic estimates regarding charge exchange between the surface and the projectile ion are consistent with the relative amounts of chemically sputtered products observed for each of the surfaces.