Non-adiabatic reaction pathways in the dissociative adsorption of Oxygen on an Al(111) surface

In the present work the results of an experimental study on the inital stages of alumi-num oxidation are reported. Despite a long-standing theoretical and experimental effort, this process still presents some puzzling characteristics. Among them, the direct, activated character of the chemi-sorption process: the initial sticking coefficient S 0 is approximately 1% for thermal mole-cules, E i = 0.025 eV, but rises to 90% at E i = 0.9 eV. This findings are at variance with the results of recent density functional theory cal-culations, predicting near unity reaction probability, even at low E i . In an attempt to clar-ify the dynamics of the initial stages of the oxidation process, I investigated the O 2 /Al interaction by means of molecular beam and laser spectrometric techniques (resonantly enhanced multiphoton ionization - REMPI). The results of the present work, coupled to the finding of scanning tunneling microscopy investigations performed by A.J. Komrowski and A.C. Kummel of the University of California, provide compelling evi-dence for the existence of an abstractive pathway for the dissociation of oxygen on alu-minum. The REMPI study also allowed to highlight the dependence of the abstraction coeffi-cient on both the translational and rotational energy of the incoming oxygen molecules.

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