Reactive ion scattering from surfaces rearing isomeric chlorinated adsorbates

The value of low-energy ionic collisions for selective surface analysis is shown by the fact that reactive scattering allows differentiation of isomer ic chemisorbates. Reactions of Cr+ and Cr-containing cations at chlorobenzy l mercaptan (CBM) monolayers on Au surfaces show different reaction product s, depending on the position of chlorine substitution in the phenyl ring. T he 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 sensiti vity of reactive ion scattering to the isomeric adsorbate is further demons trated 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-C BM, 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 relation ship applies for the dehydrohalogenation product C7H5Cr+ where the Cl atom at the ortho position in 2-CBM facilitates the intramolecular elimination o f HCl. The 85eV Xe.+ chemical sputtering mass spectra are the same for both surfaces, indicating similarities in the electron-transfer and momentum-tr ansfer processes. A related set of isomers, 2-, 3- and 4-chlorothiophenol, chemisorbed on gold, shows no differences in ion/surface reactions or chemi cal sputtering.