Interpretation of the carbon Auger line shapes for the adsorption and decomposition of ethylene on Ni(100)

The CKVV Auger line shapes of ethylene chemisorbed on Ni(100) at 100, 250, 300 and 600 K are reported and consistently interpreted. These line shapes are representative of π-bonded ethylene (C₂H_4(a)) at 100 K, σ-bonded vinyl (CHCH_2(a)) at 250 K, a mixture of ethynyl (CCH_(a)), methylidyne (CH_(a)) and di-σ-bonded acetylene (HCCH_(a)) at 300 K, and carbidic carbon (C_(a)) at 600 K on the surface. The Auger line shape of graphitic carbon adlayers on Ni is also interpreted. The line shapes are found to consist of adsorbate-adsorbate, adsorbate-substrate, and substrate-substrate components, which are denoted by the final location of the two holes created by the Auger process. The adsorbate-adsorbate component, for the C₂ chemisorbed species, reflects primarily the valence density of states of ethylene and shows negligible hole-hole correlation effects due to electron transfer from the metal. This is in contrast to the graphitic adlayer where the adsorbate-adsorbate component has the same line shape as bulk graphite and exhibits significant correlation effects. The adsorbate-substrate and substrate-substrate components seen in all of the line shapes directly reflect the adsorbate-substrate π or σ bonding character, and the extent of screening via charge transfer from the metal to the core excited adsorbate.