Effects of electron irradiation on structure and bonding of SF6 on Ru(0001)

Abstract

Electron-stimulated desorption ion angular distribution (ESDIAD) and temperature-programmed desorption (TPD) techniques have been employed to study radiation-induced decomposition of fractional monolayer SF6 films physisorbed on Ru(0001) at 25 K. Our focus is on the origin of F+ and F− ions, which dominate ESD from fractional monolayers. F− ions escape only in off-normal directions and originate from undissociated molecules. The origins of F+ ions are more complicated. The F+ ions from electron-stimulated desorption of molecularly adsorbed SF6 desorb in off-normal directions, in symmetric ESDIAD patterns. Electron beam exposure leads to formation of SFx (x=0–5) fragments, which become the source of positive ions in normal and off-normal directions. Electron exposure >1016 cm−2 results in decomposition of the entire adsorbed SF6 layer.

Electron-driven chemistry of halogenated compounds in condensed phases: Effects of solvent matrices on reaction dynamics and kinetics

“…The ESDIAD detector is used to measure angular distributions of desorbed positive and negative ions, and has TOF capability for mass-resolved ion detection. The details of these techniques have been reported elsewhere [36,40,41].…”

“…The experiments that are the focus of this article were carried out in three different ultrahigh vacuum (UHV) systems [36][37][38][39][40][41]. Figure 1 represents the schematic of a virtual (generalized) set-up that illustrates the instrumentation used in these chambers for characterization of the surface structure, composition and reactivity of thin films containing halogenated compounds.…”

“…The azimuthal orientations of S-F bonds for SF 6 adsorbed on Ru(0001) at 25 K have been established by ESDIAD [40,41]. After deposition of submonolayer coverages of SF 6 , strong off-normal emissions of both F þ and F À ions are observed.…”

“…The central beam exists only for F þ ions (figure 2(b)) and is due to ESD Electron-driven chemistry of condensed halogenated compounds from a thermal decomposition product of SF 6 , either chemisorbed F or another fragment with an S-F bond perpendicular to the surface. Electron beam exposures higher than 10 14 cm À2 lead to formation of SF x (x ¼ 0-5) fragments, which become the source of positive ions in normal and off-normal directions [40]; exposures >10 16 cm À2 result in decomposition of the entire adsorbed SF 6 layer. Figure 3(a) shows an ESDIAD pattern for F À ions from a fractional monolayer of CF 2 Cl 2 deposited at 25 K on the same substrate.…”

“…This difference is attributed to the low kinetic energies of Cl À ions, which results in the majority of the Cl À ions remaining trapped on the surface as they have insufficient kinetic energies to overcome the polarization barrier. For positive ions, the dominant gas-phase species produced during electron-stimulated reactions of condensed CF 2 Cl 2 over a wide range of electron energies ($40 to >200 eV) include CF 2 Cl þ , Cl þ and F þ [71].…”