No abstract

No abstract

Tennessee Eastman Co., and Dow Coming Corp. for their generous support under the Presidential Young Investigator Award (Grant No. CPE-8351190). We thank Prof. C. B. Richardson of the University of Arkansas for his technical advice.

Hl value. As shown in Figure 5, the shape of the qst curve on OG-400 and OG-25 is very similar to that on OG-700, the only difference being the fact that the break point slightly shifts to higher coverage of H20 from OG-700 to OG-400 and further to OG-25. Admittedly, there is a great difference between the qst curves on OG-700 and OG-IOOO.The reason why such a difference appears can be attributed to the fact that the CO content on OG-700 is as much as those on OG-25 and OG-400, in contrast to a smaller value on OG-IOOO (Table I). Walker and Janov have reported that the CO-desorbing oxides on Graphon act as the sites for adsorption of H20, which give the heat of adsorption similar to the HL value.22 Thus, the H20 molecules will be adsorbed on the CO-desorbing oxides of OG-25, OG-400, and OG-700 after the sites, such as the C02-and H20-desorbing oxides, have been completely covered with H20.thank the Ministry of Education (Japan) for the financial support granted for this research.

Adsorption of L-DOPA and a series of other amino acids and related compounds from aqueous solutions at well-defined Pt(lll) and Pt(100) single-crystal surfaces has been studied: 3-(3,4-dihydroxyphenyl)-Lalanine (DOPA), L-tyrosine (TYR), L-cysteine (CYS), L-phenylalanine (PHE), L-alanine (ALA), dopamine (DA), catechol (CT), and (3,4-dihydroxyphenyl)acetic acid (DOPAC). Packing densities (moles adsorbed per unit area) were measured for each compound by means of quantitative Auger spectroscopy; two independent Auger spectroscopic methods were employed: one based upon measurement of elemental Auger signals from the adsorbed layer and the other upon measurement of the attenuation of the Pt Auger signal by the adsorbed layer. Packing densities of DOPA, TYR, DA, CT, and DOPAC indicate adsorption with the aromatic ring attached parallel to the surface. In contrast, CYS is adsorbed through the sulfur atom, while ALA and PHE are attached to the surface through their amino acid moieties. Vibrational spectra of the adsorbed layer formed from each of these compounds were obtained by means of electron energy-loss spectroscopy (EELS) and were compared with the infrared spectra of the parent compounds in KBr. The EELS (adsorbed layer) and IR (solid compound) spectra are similar except that the hydroxyl hydrogens of DOPA, TYR, DA, CT, and DOPAC as well as the carboxyl hydrogens of PHE and DOPAC are removed as a result of the adsorption process. EELS bands of polar groups such as O-H are not broadened to the same extent as in the IR spectra of the solid compounds, evidently due to less intermolecular bonding among such groups at the surface. LEED experiments indicate that CT forms a (3X3) adlattice at Pt(lll), while each of the other compounds forms an oriented but otherwise structurally disordered layer at Pt(lll) and Pt(100). The packing density of DOPA, CYS, and DOPAC is higher at Pt(100) than at Pt(lll) due to differences in molecular orientation. Single-crystal surface structure, adsorbate orientation, and mode of surface bonding exert a profound influence on the product distribution of electrocatalytic oxidation of these well-characterized adsorbed organic intermediates, on the basis of cyclic voltammetry and potential-step chronocoulometry experiments. This study has also revealed that evacuation does not alter the composition or electrochemical properties of the chemisorbed layer formed from solution.

No abstract

W e report angular distributions of Auger electrons (507 and 5 18 eV) emitted from three distinct monolayer structuresof iodineatomsonsingle-crystal plathum: Pt(l1 1)(d3Xd3)R30°-I, P t ( l l l ) ( d 7 X d7)R19.1°-I, and Pt( 11 1)(3X3)-1. Each of these monolayers has been characterized by means of LEED, STM, AES, XPS, TDMS, and electrochemistry. Accordingly, these monolayers provide an opportunity to explore the nature of Auger electron angular distributions for surface atomic layers of known structure and to correlate the results with what is already known regarding these structures. Evident near grazing angles of emission in the measured angular distributions are intensity minima along trajectories corresponding to the 1-1 internuclear directions and intensity maxima along trajectories corresponding to the gaps between neighboring iodine atoms. Visible near the surface normal are features due to electrons emanating from the Pt(ll1) substrate. Simulations of the measured angular distributions in which iodine atoms are treated as point emitters and spherical scatterers of Auger electrons are in qualitative agreement with experiment. These results suggest that iodine atoms act predominantly to block Auger electrons in this energy range, producing minima in the angular distributions.Accordingly, the locations of the minima are a direct consequence of the relative positions of atoms, making measurements of Auger electron angular distributions useful for real-space probing of surface atomic and monolayer structure. IntroductionIn recent years, there has been increased interest in measurements capable of revealing the arrangement of atoms at or near the surface of a solid. The more productive approaches have been those based upon the use of multiple techniques, yielding complementary information useful for resolving ambiguities that might otherwise have remained. The present study begins with low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and angular distribution Auger measurements (ADAM) for a series of well-known monola er structures: Pt-(111)(d3Xd3)R30°-I, P t ( l l l ) ( d 7 X~? ) R 1 9 . 1~-1 , and Pt(111)(3X3)-I.1-9 The LEED and Auger results are then compared to recent STM results obtained for the same structure^.^-^ An important outcome of this study is that the ADAM results confirm the findings obtained by the other methods. In addition, they also provide quantitative support for the corrugated structures recently revealed by STM.6-9Auger electron a'ngular distributions are obtainedlk12 by excitation of the sample with an electron beam at fixed direction of incidence while a moving analyzer measures the angular distribution for a specific Auger electron kinetic energy. "Complete" distributions are measured and displayed over the full range of angles of emission so that the angular distribution and surface structure can be readily visualized. Recent measurements of complete distributions have led to the o b~e r v a t i o n l~'~ that intensity minima are present along interatomic dire...

Measurements of the complete angular distribution of Auger electrons emitted from well-defined platinum[111] single-crystal surfaces have led to the discovery that the distributions are composed of "silhouettes" of surface atoms "back lit" by emission from atoms deeper in the solid. Theoretical simulations of Auger electron angular distributions based upon atomic point emitters and spherical atomic scatterers of uniform cross section are in close agreement with these experimental results, but opposite to previous theoretical predictions. In view of the definitive results obtained and the straightforward agreement between theory and experiment, angular distribution Auger microscopy (ADAM) is useful for direct imaging of interfacial structure and investigation of electron-solid interactions in the physical and biological sciences and engineering. Applicability of ADAM is illustrated by images obtained for monolayers of silver and iodine on platinum[111].