SURFACE-REACTIONS BETWEEN O-2 AND HYDROCARBONS INDUCED BY DISSOCIATIVE ELECTRON-ATTACHMENT

Desorption of O- and OH- ions induced by low-energy (4-20 eV) electron impact on O2 and hydrocarbon molecules (C nH2n+2, n=5 and 8; CnH2n, n=2, 3, and 4) coadsorbed on Pt is reported. The magnitude of the O- and OH- signals is investigated as a function of incident electron energy and substrate coverage. Beyond monolayer coverage, results are provided for two types of coadsorption: a single hydrocarbon layer physisorbed on a multilayer O2 film and a multilayer film containing 25% volume O2 mixed with hydrocarbon molecules. For all experiments, the OH- yield function can be correlated with that of the O- signal from pure O2 and hydrocarbon-O2 mixture films. This result indicates that the OH- ions are produced by the abstraction reactions O -+CnH2n+2→OH-+C nH2n+1 and O-+CnH 2n→OH-+CnH2n-1, where O - ions are generated by the dissociative attachment reaction e+O 2 (3Σg-)→O 2-(2Πu,2Σ g+,2Σu+) →O-(2P)+O(3P,1D). The observed reaction efficiency for OH- formation, defined as the ratio of the OH- intensity to that of O-, is found to increase with coverage of the substrate by C4H8-O2 and C 5H12-O2 mixtures. It reaches values of 3% and 8%, respectively, above 3 monolayers for incident electrons of 13 eV. The energetics involved in those reactions as well as the behavior of the OH - intensity as a function of incident electron energy and coverage strongly suggest that OH- arises from dissociation of the intermediate quasi-bound anions CnH2n+2O- and CnH2nO- into the limits OH-+C nH2n±1. © 1990 American Institute of Physics.