Dissertation

Structural analysis of methyl sulfide/nickel(111) and methyl sulfide/nickel(001) using angle-resolved photoemission

Tang, Tianhua

1996

Overview

The investigation of a solid surface structure is an important field of research in science. To understand chemical adsorption or catalytic processes on the surface, more detailed investigations of the properties of solid surfaces are needed. This dissertation addresses the theory and applications of an important surface analysis technique: x-ray photoelectron diffraction (XPD) (126). A full multiple-scattering scheme for the calculations of x-ray electron diffraction spectra from a surface cluster is described. This strategy forms the basis of the concentric-shell algorithm. In that scheme, the calculations are rendered tractable by the division of the cluster into concentric shells centered on the photoemitter (45). X-ray photoelectron diffraction will he used to firmly establish the adsorption geometry of CH$\\sb3$S/Ni(111) and CH$\\sb3$S/Ni(001) by comparing the experimentally measured photoelectron angular distribution with theoretically calculated distribution for various trial structures. The key to success for this strategy is to have effective methods for accurately calculating the structures of the adsorbed species from the angular distributions of the photoelectrons. This method is called a full multiple-scattering (FMS) algorithm which was proposed by D. K. Saidin et al. (45). We also address the structural analysis of S on Ni(111) and Ni(001) by computer simulating the angle-resolved photoemission process. The theoretical photoemission angular dependent spectrum was compared with the experimental photoemission angular dependent spectrum from atomic S which is known to adsorb in the FCC hollow site on Ni(111) and in the four fold hollow site on Ni(001) (131). This work was initially motivated by the desire to determine the geometrical structure of these species and also to compare the spectrum obtained by experiment and theory to test our theory and program. Photoelectron diffraction from S 2p core levels has been used to determine the adsorption site and orientation of sulfur and methyl thiolate (CH$\\sb3$S) on Ni(111) and Ni(001) (8). The purpose of study S/Ni(111) is to determine the perpendicular distance of sulfur to the substrate. At 150K, CH$\\sb3$S adsorbs in the FCC hollow site on Ni(111) and in the four fold site in Ni(001). The C-S bond is tilted from the surface normal on Ni(111) and is normal to the surface on Ni(001). Therefore, the purpose for studying these species is to determine the bond length of carbon and sulfur, the perpendicular distance of sulfur to the substrate as well as to determine the orientation of them. Upon annealing to 250K, CH$\\sb3$S changes adsorptional orientation on Ni(111). The C-S bond becomes orientated along the surface normal. The purpose of studying this species is, therefore, to determine the bond length of carbon and sulfur as well as the perpendicular distance of sulfur to the substrate. In recent studies, the CH$\\sb3$S/Ni(111) is considered to be a reconstructed surface (139). The investigation of these suggested models are also addressed in the dissertation. As is the case with low energy electrons employed in other surface probes, photoelectrons scatter strongly form atoms within the sample. Consequently, the accurate computer simulation of photoelectron diffraction pattern is a challenge of multiple-scattering theory. The excellent agreement between the experimental data and theoretical data calculated in this work suggests that the calculation of the angular distribution can adequately describe the experiment and offers a powerful tool for establishing the nature of adsorbed species on chemically heterogeneous surface.

• Publisher: ProQuest Dissertations & Theses
• Subject: Atomic physics / Chemistry / Condensation / Condensed matter physics / Physical chemistry
• ISBN: 9780591033328, 0591033321