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 (E_i) 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 E_i≂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.