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 energy range (0-175 eV). ESD from adsorbed PF, gives rise to several negative ion fragments: F- (predominantly), F-2(-), P-, and PF-. The F- yield produced in the electron energy range 0-15 eV proceeds via dissociative electron attachment (DEA); the F- yield exhibits a peak around 11.5 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 electron 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 onset 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- signal 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 of negative ions from adsorbed PF3. We suggest that Rydberg core-excited (1-hole, 2-electron) transient anion states of PF3, formed by capture of low energy electrons, dissociate to produce the F- and F-2(-) ions for E(e)<15 eV. (C) 1996 American Institute of Physics.