Dissociative attachment reactions in electron stimulated desorption from condensed O2 and O2-doped rare-gas matrices

Abstract

Desorption of the ions O−, O−2,O−3 (and/or O2⋅O−) induced by electron impact on pure O2 multilayer films and Ar, Kr, and Xe matrix films containing O2 is reported. In addition to these anions, the ionic complexes M⋅O− (M=Ar and Kr) are also observed to desorb from Ar and Kr matrices, respectively. In the range 4–16 eV, the incident electron energy (Ei) dependence of the yields (i.e., the yield functions) of all the diatomic and triatomic anions exhibit features which can be correlated with the O− yield function; indicating that, these anions are produced by dissociative attachment reactions whose first step involves the formation of O−2 quasibound states. From analysis of all yield functions and variations of the anion yields as a function of O2 concentration in the matrices, we find that the simplest dissociative transient state, which can propel in vacuum an M⋅O− or O2⋅O− ion, must have the configuration M⋅O2⋅O−*2. To explain the formation of O−2 and O−3 ions below Ei≂6 eV, the existence of an electronically excited O−4 state decaying into the limits O−2+O2 and O−3+O must be postulated. At higher energies, O−2 can be formed by the reaction of O− (produced by dissociating O−2 states) with other O2 molecules (e.g., O−+O2→O−3→O−2+O). Both transient anion, M⋅O2O−*2,O−*4 result from initial electron capture by an O2 molecule in a dimeric configuration.