Adsorbate geometry distinction in arenethiols by ion surface reactive collisions

Reactive scattering of low-energy ions from surfaces gives scattered produc t 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 sp uttering induced by Xe.+, are used to distinguished two well-characterized monolayer systems, namely 1,4-benzenedimethanethiol (BDMT) adsorbed on Au(1 11) and Ag(lll) thin films. While the reaction of Cr.+ with the Au monolaye r produces an ion assigned as CrC7H5S+, this product is completely absent u pon reaction with the Ag monolayer. Pyridine (C5H5N.+) projectiles abstract C-1-C-8 hydrocarbon groups in 50 eV collisions with the Au monolayer, whil e the Ag monolayer shows only C-1-C-4 abstraction with a significantly diff erent intensity pattern. Chemical sputtering (Xe.+) mass spectra of the two surfaces an substantially different; complete fragmentation of the Ae adso rbate occurs, leading to C-2-C-4 ion ejection, while more of the molecular features are preserved in the spectrum recorded for the Au monolayer. The e xperimental facts are interpreted in terms of the geometry of the BDMT mole cule 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 rea ctive collisions are useful for characterizing chemisorbates, including the ir geometrical orientation.