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College Park, Maryland      June 6 - 10 , 2004

WP11: Neutron and Thermodtnamic investigations of H2 on MgO Surfaces

T. Arnold (Oak Ridge National Laboratory), L. Frazier (University of Tennessee), A. Ramirez-Cuesta (ISIS, Rutherford Appleton Laboratory), R. Hinde (University of Tennessee), J. Z. Larese (Oak Ridge National Laboratory; University of Tennessee)

We will discuss recent thermodynamic and neutron scattering measurements that probe the growth of molecular hydrogen films on MgO (100) surfaces. Our thermodynamic investigations indicate that at temperatures in the neighborhood of 10K, 6 or 7 layering transitions are easily observable. Using inelastic neutron techniques we have identified a feature in the spectrum that we believe can be associated with the ortho-para rotational transition in the neighborhood of 11 meV (88cm-1). This feature is significantly lower than the equivalent transition in bulk H2 which appears at 14.7 meV. The position and shape of this feature has been recorded as a function of temperature. Distinct shifts in the peak position and symmetry are recorded between one and three layers. In the vicinity of three layers a sharp feature near the bulk transition appears and continues to grow in intensity with additional increases in the hydrogen film thickness. Comparison of these results with simple calculations and structural data will be made. Relationship of these data to others recorded when H2 is adsorbed other metal oxide materials will be discussed.

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