Reaction of water with MgO(100) surfaces. Part I : Synchrotron X-ray photoemission studies of low-defect surfaces

摘要: Abstract Synchrotron-based photoemission spectroscopy and low-energy electron diffraction were used to study the reaction of water vapor at 300 K different pressures, p (H 2 O), ranging from 5×10 −9  Torr 10 −3 (3 min exposure each pressure), with vacuum-cleaved MgO(100) surfaces that had low (5–10%) defect densities. The O 1s, Mg 2p, 2s/VB spectra acquired photon energies chosen optimize surface sensitivity. 1s are sensitive adsorption onto even very exposures [ −8 for 3 min ( −4 (≥1.8×10 4  L)]. Comparison these similar immersed in bulk polycrystalline Mg(OH) indicates chemisorbs dissociatively two distinct stages on “low defect” surfaces, forming hydroxyl groups. first stage occurs ≤3×10 −5 (≤5.4×10 3  L) or 30 min L) involves a relatively fast defects (corner edge-step sites point defects) comprising 5–10% sites, agreement recent first-principles electronic structure calculations. second higher pressures (≥10 3 min) dissociative chemisorption terrace which is not predicted by apparent sticking coefficient (≥0.16) about four orders magnitude larger than (≥3×10 ), suggesting requires significantly more activation energy stage. Our results also suggest hydroxylation time below threshold pressure ≈10  Torr. Although both kinetic thermodynamic interpretations possible, analysis (using solid free energies) predicts approximately same as observed. After fully hydroxylated, additional can be physisorbed layer. Analysis taken but hydroxyls formed predominantly under conditions. experimental data show 4–6 eV electrons mitigate change during experiments have no effect dissociation surface.