Surface reactions between O2 and hydrocarbons induced by dissociative electron attachment

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Abstract
Desorption of O− and OH− ions induced by low-energy (4–20 eV) electron impact on O2 and hydrocarbon molecules (CnH2n+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−+CnH2n+1 and O−+CnH2n→OH−+CnH2n−1 where O− ions are generated by the dissociative attachment reaction e+O2 (3∑−g)→O−2 (2∏u,2∑+g,2∑+u)→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 C5H12–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+2 O− and CnH2n O− into the limits OH−+CnH2n±1.