Adsorbate geometry distinction in arenethiols by ion/surface reactive collisions

Abstract

Reactive scattering of low-energy ions from surfaces gives scattered product ions in which new bonds are formed with the adsorbate with a sensitivity to adsorbate geometry. Reactions of Cr^•+ and C₅H₅N^•+, as well as chemical sputtering induced by Xe^•+, are used to distinguished two well-characterized monolayer systems, namely 1,4-benzenedimethanethiol (BDMT) adsorbed on Au(111) and Ag(111) thin films. While the reaction of Cr^•+ with the Au monolayer produces an ion assigned as CrC₇H₅S⁺, this product is completely absent upon reaction with the Ag monolayer. Pyridine (C₅H₅N^•+) projectiles abstract C₁-C₈ hydrocarbon groups in 50 eV collisions with the Au monolayer, while the Ag monolayer shows only C₁-C₄ abstraction with a significantly different intensity pattern. Chemical sputtering (Xe^•+) mass spectra of the two surfaces are substantially different; complete fragmentation of the Ag adsorbate occurs, leading to C₂-C₄ ion ejection, while more of the molecular features are preserved in the spectrum recorded for the Au monolayer. The experimental facts are interpreted in terms of the geometry of the BDMT molecule that is monocoordinated on Au and dicoordinated on Ag. The experiments are extended to the 1,2- and 1,3-ring isomers of 1,4-BDMT, which have not been characterized by other forms of surface spectroscopy. Ion/surface reactive collisions are useful for characterizing chemisorbates, including their geometrical orientation.