Low-energy (<100 eV) ion/surface reactions of metal ions, Ti*+, Cr*+, Fe,+, Mo*+, and W,+, at a fluorinated self-assembled monolayer surface give fluorine-containing scattered ions, MF"+, n < 5. The metal fluorides are the most abundant products in the scattered ion spectra. The single fluorine abstractions are all endothermic and driven by the projectile translational energy. Multiple fluorine abstraction occurs with high efficiency for Mo,+ and W,+ projectiles, where the thermochemistry is most favorable. Polyatomic projectile ions derived by partial fragmentation of the Cr, Mo, and W hexacarbonyls also yield metal fluoride scattered ions, in addition to more complex species formed by fluorine abstraction by ions which retain one or more carbon atoms or carbonyl groups. Fe-and Ti-containing projectile ions, bearing one or two cyclopentadienyl (Cp) groups, also exhibit fluorine abstraction products, with and without retention of a Cp group. TiCl"+ (n = 1-4) projectiles exhibit fluorine abstraction, as well. Similar fluorine abstraction products are generated upon collisions at a liquid perfluorinated polyether surface. Gas-phase ion/molecule reactions and thermochemical considerations suggest that multiple fluorine abstractions can occur by a direct reaction mechanism within a single scattering event and that multiple fluorine atoms are probably derived from a single fluorocarbon chain. Angle-resolved scattering data support a single-collision, multiple-atom abstraction mechanism. The scattered ions leave the surface with very low translational energy and collision energy variations suggest that in these ion/surface reactions, projectile dissociation is concerted with or occurs prior to fluorine abstraction. There is no evidence that electron transfer to the surface is involved, as it is in alkyl group abstraction reactions by ions at hydrocarbon surfaces.
Reactive collisions of low energy (<100-eV) mass-selected ions are used to chemically modify fluorinated self-assembled monolayer surfaces comprised of alkanethiolate chains CF3(CF2)11(CH2)2S- bound to Au. Typical experiments were done by using 1-nA/cm(2) beams and submonolayer doses of reactant ions. Characterization of the modified surface was achieved by in situ chemical sputtering (60-eV Xe(+·)) and by independent high mass resolution time-of-flight-secondary ionization mass spectrometry (TOF-SIMS) (15-25-keV, Ga(+)) experiments. Treatment with Si(35)C1 4 (+·) produced a surface from which Xe(+) sputtering liberated CF2 (35)C1(+) ions, which suggested Cl-for-F halogen exchange at the surface. Isotopic labeling studies that used Si(35)Cl2 (37)Cl 2 (+·) ; and experiments with bromine-containing and iodine-containing projectiles, confirmed this reaction. High mass resolution TOF-SIMS spectra, as well as high spatial resolution images, provided further evidence as to the existence of halogen-exchanged species at the bombarded surface. Analogous Cl-for-F halogen substitution was observed in a model gas-phase reaction. The ion-surface reaction is suggested to proceed through an intermediate fluoronium ion in which the projectile is bonded to the target molecule. The most significant conclusion of the study is that selective chemical modification of monolayer surfaces can be achieved by using reactive ion beams, which lead to new covalent bonds at the surface and in the scattered ions.
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