Date of Award
Doctor of Philosophy
John Z. Larese
T. Ffrancon Williams, Frank Vogt, Takeshi Egami
Volumetric adsorption isotherms and computational molecular dynamics (MD) simulations were performed for nonane and decane adfilms on MgO(100) nanocubes. From the isotherms, variety of thermodynamic quantities are calculated. These values, along with visual inspection of the isotherms, indicate a layer by layer trend from 2D to 3D behavior. This is attributed to the increasing importance of vertical adsorbate-adsorbate interactions as distance from the surface increases. Additionally, a 2D phase transition is observed for the first adsorbed layer as indicated by the evolution of the widths of the isotherm first derivative peaks. These experimental results are complemented by the MD calculations, which provide a qualitative description of the behavior of adsorbate molecule as temperature and surface coverage are varied. Excellent agreement is found between theory and experiment. These results are then compared to those of shorter-chain alkanes, and apparent deviations from trends relating thermodynamic properties and alkane chain length are explained in terms of molecule-surface compatibility.
In a separate investigation, inelastic neutron scattering measurements of rotational tunneling spectra of CH2D2 and CH3D are also presented in order to explore the effects of site and molecular symmetry on the rotational tunneling behavior of adsorbed methane monolayer solids. The experimental spectra are interpreted in terms of a pocket state model that accounts for these symmetries using a 12x12 Hamiltonian matrix. Diagonalizing this matrix produces a tunneling diagram that can be used to predict possible tunneling transitions. The experimental spectra are analyzed by comparing the widths and energy transfers of the observed spectral features. This analysis indicates a relatively simple spectrum for CH2D2 and a much more complicated spectrum for CH3D. These differences in complexity are explained by the complementary symmetry of the CH2D2 molecule and the interaction potential of the adsorption site as compared to the mismatch in symmetry between the CH3D molecule (threefold) and the speculation adsorption site (two-fold). Reasonably good agreement between theory and experiment is found for CH2D2/MgO, while the assumptions made in developing these models remain uncertain for the CH3D system.
Hicks, Andrew Spencer, "Alkane Adsorption on MgO(100): Volumetric Isotherm, Inelastic Neutron Scattering, and Computational Studies. " PhD diss., University of Tennessee, 2013.