Reactions of BBrn+ (n=0-2) at fluorinated and hydrocarbon self-assembled monolayer surfaces:: observations of chemical selectivity in ion-surface scattering

Ion-surface reactions involving BBrn* (n = 0-2) with a fluorinated self-assembled monolayer (F-SAM) surface were investigated using a multi-sector scattering mass spectrometer. Collisions of the B+ ion yield BF2+ at threshold energy with the simpler product ion BF+. appearing at higher collision energies and remaining of lower abundance than BF2+ at all energies examined. In addition, the reactively sputtered ion CF+ accompanies the formation of BF2+ at low collision energies. These results stand in contrast with previous data on the ion-surface reactions of atomic ions with the F-SAM surface in that the threshold and most abundant reaction products in those cases involved the abstraction of a single fluorine atom. Gas-phase enthalpy data are consistent with BF2' being the thermodynamically favored product. The fact that the abundance of BF2+ is relatively low and relatively insensitive to changes in collision energy suggests that this reaction proceeds through an entropically demanding intermediate at the vacuum-surface interface, one which involves interaction of the B+ ion simultaneously with two fluorine atoms. By contrast with the reaction of B+, the odd-electron species BBr+. reacts with the F-SAM surface to yield an abundant single-fluorine abstraction product, BBrF+. Corresponding gas-phase ion-molecule experiments involving B+ and BBr+. with C6F14 also yield the products BF+. and BF2', but only in extremely low abundances and with no preference for double fluorine abstraction. Ion-surface reactions were also investigated for BBrn+ (n = 0-2) with a hydrocarbon self-assembled monolayer (H-SAM) surface. Reaction of the BI ion and dissociative reactions of BBr+. result in the formation of BH2+, while the thermodynamically less favorable product BH+. is not observed. Collisions of BBr2+ with the H-SAM surface yield the dissociative ion-surface reaction products, BBrH+ and BBrCH3(+). Substitution of bromine atoms on the projectile by hydrogen or alkyl radicals suggests that Br atoms may be transferred to the surface in a Br-for-H or Br-for-CH3 transfer reaction in an analogous fashion to known transhalogenation reactions at the F-SAM surface. The results for the H-SAM surface stand in contrast to those for the F-SAM surface in that enhanced neutralization of the primary ions gives secondary ion signals one to two orders of magnitude smaller than those obtained when using the F-SAM surface, consistent with the relative ionization energies of the two materials. Copyright (C) 2001 John Wiley & Sons, Ltd.