Chemisorption geometry of hydrogen on Ni(111): Order and disorder

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Abstract
The location of a half monolayer of ordered hydrogen adatoms on Ni(111) has been analyzed by Low‐Energy Electron Diffraction (LEED), Thermal Desorption Spectroscopy (TDS), and Work Function (Δφ) measurements. It is found that the hydrogen atoms are arranged in an overlayer of graphitic structure with a (2×2) unit cell with respect to the substrate unit cell. In the ordered regions, the hydrogen adatoms occupy both types of three fold hollow sites without a detectable difference in the Ni–H bond lengths between the two sites. The Ni–H bond length is found to be 1.84±0.06 Å, corresponding to an overlayer‐substrate spacing of 1.15±0.1 Å. The relation between this structure and its observed order–disorder phase diagram as a function of temperature and hydrogen coverage is discussed. The disorder is discussed in detail, and a novel ’’atomic band structure’’ interpretation is given.