Reactive ion scattering from surfaces bearing isomeric chlorinated adsorbates

Abstract

The value of low-energy ionic collisions for selective surface analysis is shown by the fact that reactive scattering allows differentiation of isomeric chemisorbates. Reactions of Cr^•+ and Cr-containing cations at chlorobenzyl mercaptan (CBM) monolayers on Au surfaces show different reaction products, depending on the position of chlorine substitution in the phenyl ring. The chlorine atom abstraction product, CrCl⁺, is observed at 45-eV collision in 4-CBM and is completely absent in 2-CBM at the same energy. The sensitivity of reactive ion scattering to the isomeric adsorbate is further demonstrated by the fact that the peak corresponding to Cr^•+ addition and Cl loss via Cl-C bond cleavage, C₇H₆SCr⁺, is large in the monolayer formed from 4-CBM, but weak in the 2-CBM monolayer, suggesting that the chlorine is below the first layer of atoms in the latter case. The inverse intensity relationship applies for the dehydrohalogenation product C₇H₅Cr⁺ where the Cl atom at the ortho position in 2-CBM facilitates the intramolecular elimination of HCl. The 85-eV Xe^•+ chemical sputtering mass spectra are the same for both surfaces, indicating similarities in the electron-transfer and momentum-transfer processes. A related set of isomers, 2-, 3- and 4-chlorothiophenol, chemisorbed on gold, shows no differences in ion/surface reactions or chemical sputtering.