The adsorption of H2O and its decomposition on clean Fe(100) have been studied using low-energy electron diffraction (LEED), temperature programmed desorption (TPD), and high-resolution electron energy loss spectroscopy (EELS). Water adsorbs molecularly on the surface at 100 K and desorbs from three states at 165, 220, and 310 K. EELS and TPD data suggest that the Fe(100) surface interacts sequentially with water, forming hydrogen-bonded molecular clusters at low temperature and low coverage. As the surface is warmed, wetting occurs as the clusters break apart, and molecular water begins to dissociate. Dissociation is complete at a temperature of 250 K, forming a p(1 × 2)-OH overlayer, with the OH bond tilted from the surface normal. The hydroxyl overlayer disproportionates or decomposes resulting in water or hydrogen desorption near 310 K. Oxygen remaining on the surface following this desorption is bound in the fourfold hollow site, as has been observed for oxidation of this surface by O2.
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