The use of direct recoil spectrometry (DRS) for the study of water vapor interactions on polycrystalline metallic surfaces—the H2O/U and H2O/Ti systems |
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Authors: | MH Mintz N Shamir |
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Institution: | a Nuclear Research Center, Negev, P.O. Box 9001, Beer-Sheva, Israel b The Ben Gurion University of the Negev, Department of Nuclear Engineering, P.O. Box 653, Beer-Sheva, Israel |
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Abstract: | Using direct recoil spectrometry (DRS), the shadowing of surface H atoms by neighboring O atoms can differentiate between full and partial dissociation routes of water molecules on the surface as well as point to the geometrical arrangements of hydroxyl surface groups. The H2O/U and H2O/Ti systems were compared. It has been found that different mechanisms control the water-surface interactions in these systems.For the H2O/U system, a simple direct-collision (Langmuir-type) dissociative chemisorption controls the process. Two consecutive stages were identified: (i) below ∼70% monolayer coverage, a complete dissociation of water into oxygen ion and two H atoms, which chemisorb on the remaining unreacted metallic surface and (ii) above about 70% of a full layer coverage, three dimensional oxide islands start to form, causing partial dissociation of water and the formation of surface hydroxyls.For the H2O/Ti system, a more complicated mechanism, which involves a precursor state, seems to control the process. In that case, two concurrent routes act simultaneously. In addition to the simple direct-collision mechanism, water precursor clusters (bound by hydrogen bonds), which partly dissociate, result in chemisorbed tilted hydroxyl clusters (even at low-coverage). The relative contributions of the precursor route and the direct-collision route are pressure dependent, with the former being dominant at higher exposure pressures. |
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Keywords: | DRS H2O/U system H2O/Ti system H shadowing |
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