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UBC Theses and Dissertations

Structure and stability of quantum H2 films on ionic surfaces : a theoretical study Sadeghifar, Alireza


The process of H₂ adsorption on ionic surfaces has long been of importance technologically as well as of interest to “pure” science because they constitute simple ideal systems that may be subject to quantum effects. Experiments have shown that these systems can form a sequence of structures that undergo phase transitions. In particular, there is evidence that H₂/MgO(100) exhibits the following sequence of structural phase transitions: c(2×2) → c(4×2) → c(6×2). The details of these structures and their stabilities are not yet understood. This thesis presents an investigation of the H₂/NaCl(100) and H₂/MgO(100) systems where the detailed structures of these films and their stabilities are determined through the use of Monte Carlo simulations in the canonical ensemble. Key to this study is a hierarchy of newly developed H₂-H₂ and H₂–surface interaction potentials that depend on the quantum mechanical rotational state of the molecule(s) and are presented here for the first time. Using these potentials, it was found that the structure of the H₂ films depend on whether the molecules were in the para (J=0) or ortho (J=1) rotational state, or alternatively consisted of a mixture of molecules in these different rotational states. In addition, the external vibrational frequencies of the H₂ molecules with respect to the surface were calculated. Overall, the structure, stability, and external vibrational frequencies of the H₂/NaCl(100) and H₂/MgO(100) systems were found to match or be consistent with experimental results. They also provided details, e.g. molecular locations, that are not accessible experimentally. Consequently, a more complete and coherent picture of the H₂/NaCl(100) and H₂/MgO(100) systems has emerged.

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