Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 1977 / Includes bibliographical references. / Ph. D. / Ph. D. Massachusetts Institute of Technology, Department of Physics

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Utilizando a Teoria do Funcional da Densidade junto com o formalismo do pseudopotencial de primeiros princípios, realizamos um estudo sistemático do processo de adsorção da molécula de oxigênio sobre a superfície livre de CdTe (110) nas reconstruções 1x1, 1x2 e 2x1. Este estudo consistiu na determinação das adsorções energeticamente favoráveis e na viabilidade de suas formações através da análise das barreiras de ativação. Nossas análises indicam que apenas uma molécula de oxigênio adsorve sobre a superfície livre em uma reconstrução 1x1 e que não ocorre a quebra da molécula durante o processo de adsorção. As estruturas formadas foram divididas nos regimes de baixas e altas temperaturas. Do estudo das barreiras de ativação verificamos que no regime de baixas temperaturas a molécula de oxigênio liga-se exclusivamente ao Cd da primeira camada através de ligações Cd-O-O ou Cd-O2. A configuração da superfície de CdTe com a molécula adsorvida, se assemelha a configuração do cristal. As estruturas de faixas de energia neste regime apresentam um estado característico de defeito duplo aceitador. Para o regime de altas temperaturas, a molécula adsorve entre o Cd da primeira camada e o Te da segunda camada, quebrando esta ligação Cd-Te e também quebrando a ligação que o Cd da segunda camada realiza com o Te da terceira camada. O complexo formado apresenta ligações Cd-O, Cd-O2, Te-O e O-O e as estruturas de faixas de energia apresentam um gap indireto entre os pontos gama e X de 1.30 eV. / Density Functional Theory with first-principles pseudopotential formalism have been used to a systematic research of the oxygen molecule adsorption on the free surface of CdTe(110) in the 1x1, 1x2 and 2x1 reconstructions. This research was based on the determination of the adsorptions energetically favorables and the viability of each formation through their activation barriers analysis. This analysis indicates that only one oxygen molecule adsorbs over the free surface of the CdTe(110) in the 1x1 reconstruction and that the dissociation of the molecule does not occur during the adsorption process. The structures were divided on the high and low temperature regimes. From the activation barriers study it was verified that on the regime of low temperatures the oxygen molecule bind exclusively to the Cd of the first layer through the Cd-O-O or Cd-O$_2$ bonds. The CdTe surface configuration with the adsorbed molecule looks like a crystal configuration. The energy band structure, on this regime, shows a characteristic state of double acceptor defect. For the high temperature regime the molecule adsorbs between the Cd of the first layer and the Te of second layer, breaking the Cd-Te bond between them and also breaking the bonds that the Cd of the second layer does with the third layer. The complex shows Cd-O, Cd-O2, Te-O and O-O bonds and the energy band structure shows an indirect gap between the gamma and X points of 1.30eV.

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The chemisorption and electrochemical hydrogenation of hydroquinone (H2Q) at polycrystalline (pc) Pd, well-ordered Pd(100), and Pd-modified Au(hkl) electrodes were studied using a combination of ultra-high vacuum (UHV) surface spectroscopy, electrochemistry (EC), and electrochemical mass spectrometry (EC-MS). H2Q was found to form a slightly tilted flat-oriented quinone (Q) adlayer, when adsorbed from low concentrations; when chemisorbed from high concentrations, an edgewise-oriented H2Q adlayer was indicated. The hydrogenation of the chemisorbed layer is initiated at potentials before the onset of the hydrogen evolution region. As expected, the kinetics increases as the applied potential is increased, but the hydrogenation pathway appears to be independent of the potential. Hydrogenation in the absence of absorbed hydrogen (sub-surface) was studied at ultra-thin Pd films on Au single-crystal substrates. Hydrogenation and/or potential induced desorption were established, although non-volatile and/or hydrophobic products were detected. In comparison, negative excursions with benzene-coated electrodes resulted in nothing more than potential-induced desorption of the starting material. Negative-potential electro-desorption was more facile at terraces than at steps. Vibrational spectroscopic measurements suggested that hydrogenation occurs one molecule at a time to the fullest extent that resulted in desorption of product; that is, partially hydrogenated species do not exist on the surface.

We use the first-principles molecular-dynamics¡@simulation method (MD), which is based on the density functional theory (DFT) with local-density approximation (LDA), to calculate the sticking property of a carbon atom on hydrogen covered C(100) surface. We focused on trajectories and kinetic energy transfer of the gas-phase C atom for four incident angles of =0, £k/8, £k/6 and £k/4. We find that the calculated trajectories and the kinetic energy transfer of the gas-phase atom, Cn, overall are not very sensitive to the change of the incident angle. The insensitivity of the sticking property on the incident angle may be due to a large chemisorption energy, which bends the trajectory of Cn toward the surface, so that Cn is confined to move within a small range.

The chemisorption and anodic oxidation of hydroquinone (H2Q) and benzoquinone (BQ) at palladium electrode surfaces was studied by a combination of electrochemistry (EC), Auger electron spectroscopy (AES), high-resolution electron- energy loss spectroscopy (HREELS) and electrochemical-scanning tunneling microscopy (EC-STM) on a smooth polycrystalline and well-defined (single-crystalline) Pd(100) electrode surface. The results point to the following more critical conclusions: (i) Chemisorption of H2Q from dilute (less than or equal to 0.1 mM) aqueous solutions forms surface- coordinated BQ oriented parallel albeit with a slight tilt. (ii) At high concentrations (greater than or equal to 1mM), chemisorption yields an edge-vertical oriented diphenolic species. (iii) The extent of anodic oxidation of the chemisorbed organic strongly depends upon its initial orientation; only the flat-adsorbed species are oxidized completely to carbon dioxide. (iv) The rate of anodic oxidation is likewise dependent upon the initial adsorbate orientation; the rate for vertically-oriented species is more than twice that of flat- adsorbed species. (v) The chemisorbed species are not oxidized (to the same extent) simultaneously; instead, oxidation occurs one molecule at a time. That is, molecules that survive the anodic oxidation and remain on the surface retain their original identities.

Thesis (Ph. D.)--Institute of Paper Chemistry, 1976. / Bibliography: leaves 110-114.