Surface science studies of Cobalt and Rhodium single crystal surfaces
Abstract (Summary)The main topic of this thesis is the investigation of small molecules adsorbed on the transition metals cobalt and rhodium surfaces by means of predominantly high-resolution core level photoemission and near edge x-ray absorption fine structure (NEXAFS). The thesis can be divided into three parts where the following phenomena are examined:1) internal molecular vibrations in the core level photoemission spectra2) hybridisation and thermal decomposition of adsorbates3) growth and surface alloy formation of metal-on-metal systemsThe influence on the C 1s spectra of photoemission induced internal vibrations after chemisorption is presented for two molecules, acetylene (C2H2) and carbonmonoxide (CO). In the former case the fine structure of the C-H vibrational stretch mode of acetylene on Co(0001) single crystal surface is resolved. The measured vibrational energy splitting is (389 ± 8) meV, which is ~6 % lower than what has been reported for gaseous acetylene. The corresponding Sfactor of 0.17 ± 0.02 is considerably higher than those reported for other systems. For CO molecules adsorbed on metal surfaces the C-O stretch vibration has a vibrational energy which is about half that of the C-H stretch in hydrocarbons. The progression of this vibrational mode for the systems CO/Co(0001), CO/Rh(100) and CO/Rh(111) have been thoroughly investigated. CO adsorbs in on-top sites on Co(0001) resulting in a vibrational splitting of (210 ± 3) meV from the adiabatic C 1s peak. For CO on Rh(100) two adsorption sites, on-top and bridge, are populated. Similar analyses of the vibrational fine structure gives a vibrational splitting of (221 ± 4) meV for on-top site and (174 ± 11) meV for bridge site. Investigations of the C-O vibrational properties on Rh(111), where CO populates on-top and three-fold hollow sites, show the same trend as for CO on Rh(100), namely decreasing vibrational splitting with increasing co-ordination. A tendency of increasing S-factor with increasing co-ordination is also reported.Acetylene hybridisation and thermal decomposition on two crystal orientation of cobalt, Co(0001) and Co(1120) , have been investigated using a variety of experimental techniques: NEXAFS shows that acetylene chemisorb strongly to both cobalt surfaces. For the system C2H2/Co(0001) a state of hybridisation close to sp3 has been deduced, which is considerably stronger than what has been reported earlier. Photoemission spectroscopy measurements show that acetylene starts to decompose at a temperature ~100 K lower for Co(1120) compared to Co(0001). For the anisotropic Co(1120) surface the acetylene decomposition lead to formation of an ordered carbon overlayer. By combining these results with STM and LEED measurements a hard sphere model has been proposed for this overlayer.Rare earth metals induce solid state reactions when thin overlayers are deposited onto a variety of metals and followed by annealing. Using high-resolution photoemission spectroscopy, LEED and temperature programmed desorption (TPD) a true surface alloy is shown to form with new electronic states when La on Rh(100) is annealed to 1350 K. These states alter the hybridisation of CO significantly compared to clean Rh(100) and as-deposited La on Rh(100). TPD experiments show CO desorption peaks at significantly lower temperature than CO on the as-deposited La/Rh(100), but still higher than the desorption peaks found for CO on Rh(100).The homoepitaxial growth of cobalt on Co(1120) has been studied by STM. The diffusion of Co adatoms is found to be highly anisotropic with larger mobility of added Co atoms parallel than perpendicular to , i.e. along the zigzag rows of the unreconstructed Co(1120) surface.
Source Type:Master's Thesis
Date of Publication:01/01/2001