Photoemission studies of inorganic (CO, O, NO) and organic (C₂H₂, C₂H₄, C₆H₆) adsorbates on Ni(111) and surface reactions

We describe adsorption studies of serveral inorganic and organic species on single crystal Ni(111) using ultraviolet photoemission spectroscopy (hv = 21.2 eV). Adsorbate orbital ionization energies and line shapes have been measured and surface reactions have been studied. Ionization energies for chemisorbed unsaturated hydrocarbons (C2H2, C2H4, and C6H6) exhibit large surface-induced relaxation shifts (- 1-3 eV) relative to their gas phase counterparts as well as π-orbital bonding shifts (- 0.9-1.5 eV). We estimate these n-d bonding interaction strengths and chemisorption energies using Mulliken’s donor-acceptor theory as described by Grimley for weak chemical bonds and show that an observed surface reaction, i.e. the dehydrogenation of chemisorbed ethylene (C2H4) to chemisorbed acetylene (C2H2) for T≳230 K, becomes exothermic only for the chemisorbed species due to the π-d electron interaction. © 1974 The Japan Society of Applied Physics.