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 C 5 H 5 N •+ , 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 CrC 7 H 5 S + , this product is completely absent upon reaction with the Ag monolayer. Pyridine (C 5 H 5 N •+ ) projectiles abstract C 1 -C 8 hydrocarbon groups in 50 eV collisions with the Au monolayer, while the Ag monolayer shows only C 1 -C 4 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 C 2 -C 4 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.

Collisions of ions with surfaces at chemically relevant energies: Instrumentation and phenomena

“…Different reaction products formed by collisions of Cr ϩ• and C 5 H 5 N ϩ , as well as chemical sputtering induced by Xe ϩ. helped to elucidate the 1,2-and 1,4-BDMT geometry on both Au and Ag surfaces. 202 Finally, as discussed previously, abstraction reactions from surfaces are typically indicative of the nature of the surface. These abstraction reactions, whether of hydrocarbon 91,202 or fluorocarbon groups, 180 provide detailed information about the surface.…”

“…Similar conclusions were reached when low energy reactive ion/ surface collisions were used to investigate the adsorption geometry of 1,2-1,3-, and 1,4-benzenedimethanethiol SAM surfaces. 202 Characteristic differences in the scattered ion products collected from each SAM were used to elucidate the adsorption geometries.…”

“…142,170,247 Recent evidence has shown that pyridine molecular ions are capable of abstracting hydrocarbon groups ͑from C 1 to C 8 ͒ from SAM surfaces, and this abstraction reveals a dependence on adsorbate structure. 202 Abstraction of a fluorine atom or a fluorocarbon group from an F-SAM surface is another well-studied ion/surface reaction. This reaction differs from hydrogen atom pickup, in that both even-electron cations and radical cations can abstract fluorine atom͑s͒.…”

Electrochemical characterization of a 1,8-octanedithiol self-assembled monolayer (ODT-SAM) on a Au(111) single crystal electrode

“…Bilayers and multilayers have been built by the oxidative formation of a S-S bond between dithiols, although these reactions depend on the presence of oxidants in the solution [10][11][12][13][14][15][16]. The addition of a reductor as tri-n-butylphosphine prevents the formation of multilayers and allows the generation of well-ordered monolayers of the dithiol in which the molecules adopt a standing-up orientation [17].…”

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

“…For example, adsorbed chlorobenzyl mercaptan isomers [5] can be differentiated based on reactions with Cr •+ , which generate such product ions as CrCl + and CrC 7 H 6 Cl + with the accessible chlorine in the p-chloro isomer. In another study [26], the molecular ion of pyridine, C 5 H 5 N •+ , was shown to allow distinction between 1,4-benzenedimethanethiol adsorbed on gold versus silver due to differences in the orientation of the adsorbates on the two metals.…”