Differentiation of isomeric oxygenated adsorbates using low-energy ion/surface collisions

Reactive scattering of polyatomic ions in the hyperthermal collision energy range (<100 eV) is used to distinguish isomeric oxygenated adsorbates and to quantify their relative amounts when co-adsorbed at a surface. The self-assembled monolayers (SAMs) of interest are constructed from HO-terminated, CH3O-terminated, and CH3CH2O-terminated dialkyl disulfides. Projectile ions used for ion/surface scattering experiments include CF3+, SiCl3+, and the molecular ion of pyridine, C5H5N[radical sign]+. Each of these ions exhibits a unique scattered ion profile upon collision with the SAM monolayer surfaces, and so provides different information about the surfaces. Hydrogen atom abstraction by the C5H5N[radical sign]+ ion is more prominent at the CH3CH2O- and CH3O-terminated surfaces than the HO-terminated surface, while collisions of SiCl3+ yield reactively scattered products which reflect the chemical composition of these surfaces. For instance, SiCl2OH+ and SiCl2OCH3+ are scattered from the HO-terminated and CH3O-terminated surfaces, respectively. Ion/surface collisions involving the CF3+ ion produce chemically sputtered ions from the oxygenated adsorbates, which are valuable for quantitation of those groups. Preferential sputtering of the CH3O-terminated versus the HO-terminated SAM surface is ascribed to favored thermochemistry and the more accessible CH3O-terminated adsorbate. Fundamental ion/surface scattering processes, such as inelastic collisions leading to surface-induced dissociation (SID), ion/surface reaction, and chemical sputtering are examined over a range of collision energies for each of the ion/surface types mentioned, and their value in surface analysis is demonstrated.