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/surfac e reactions in the course of low-energy (20-90 eV) collisions with hydroxyl -terminated (HO-SAM), hydrocarbon (H-SAM), and fluorocarbon (F-SAM) self-as sembled monolayer surfaces. Formation of the substitution product, SiCl2F+, upon collision of SiCl3+ With the F-SAM surface is the result of a transha logenation reaction. In an analogous fashion, one observes substitution of a chlorine in the SiCl3+ projectile ion by either an OH group from the HO-S AM surface or a CH3 group from the H-SAM surface to form the scattered reac tion products, SiCl2OH+ and SiCl2CH3+, respectively. The concomitant transf er of a Cl atom from the projectile ion into the surface is indicated by th e sputtered ion, CH2Cl+. The scattered product SiCl(OH)(2)(+) involves disu bstitution, 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 bo nds at the appropriate surface, and they occur after, or in concert with, s urface-induced dissociation of the polyatomic projectile. Surface effects o n the dissociation of projectile ions are studied using the Si(C2H5)(4)(.+) ion, and threshold values for translational to internal energy (T double r ight 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 e nergies, (>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 d istinguish the three surfaces. Decreased neutralization at the F-SAM surfac e is associated with increased amounts of dissociatively and reactively sca ttered product ions. Thermodynamic estimates regarding charge exchange betw een the surface and the projectile ion are consistent with the relative amo unts of chemically sputtered products observed for each of the surfaces.