ELECTRON-STIMULATED DESORPTION FROM PF3 ADSORBED ON PT .2. NEGATIVE-IONS

We have studied electron stimulated desorption (ESD) of negative ions from PF, molecules adsorbed on a Pt substrate over a wide electron ene rgy range (0-175 eV). ESD from adsorbed PF, gives rise to several nega tive ion fragments: F- (predominantly), F-2(-), P-, and PF-. The F- yi eld produced in the electron energy range 0-15 eV proceeds via dissoci ative electron attachment (DEA); the F- yield exhibits a peak around 1 1.5 eV with an onset around 7.5 eV. At electron energies above similar to 15 eV, F- ions are produced via dipolar dissociation (DD). We have found that the F- ions produced from a 1 ML PF3/Pt surface via a DEA process with 11.5 eV electron impact desorb with a peak kinetic energy of similar to 0.7 eV, while the F- ions generated via DD by 175 eV el ectron impact desorb with a peak kinetic energy of similar to 1.2 eV. The F-2(-) yield-curve also shows a peak at similar to 11.5 eV; the on set of the F-2(-) yield from adsorbed PF3 is similar to 9 eV. The F-2( -) yield in the electron range 9-15 eV is initiated via DEA. The P- si gnal from PF3 adsorbed on Pt has an onset at similar to 16 eV. We have identified some possible DEA and DD processes leading to desorption o f negative ions from adsorbed PF3. We suggest that Rydberg core-excite d (1-hole, 2-electron) transient anion states of PF3, formed by captur e of low energy electrons, dissociate to produce the F- and F-2(-) ion s for E(e)<15 eV. (C) 1996 American Institute of Physics.