The adsorption of C2H4 and C2H2 on the clean, C- and O-covered Fe(100) surface was studied using HREELS, with additional information provided by TPD, AES and LEED. On the clean surface, C2H4 adsorbed molecularly on this surface in a di-σ-bonded configuration at 100 K. The di-σ-bonded C2H4 desorbed at 240 K or decomposed to form the methylidyne (CH) and ethynyl (CCH) species below 260 K. Hydrogen is the only desorption product of C2H4 decomposition. Acetylene adsorbs on the clean Fe(100) surface in a structure with hybridization of ∼ sp3. At lower exposure (< 0.2 L), C2H2 decomposes to form CH and CCH at 253 K. At higher exposure (>0.2 L), C2H2 partially dehydrogenates and hydrogenates to form CH, CCH and CHCH2 at the adsorption temperature of 100 K. When the surface is warmed to 393 K, a CCH2 species is formed by the dehydrogenation of CHCH2. The final decomposition product of ethylene is a c(2 × 2)-C surface, formed by heating the adsorbate to 523 K. A ( 1 1 -1 2)-C surface with 0.81 ML coverage is obtained by heating the acetylene adsorbed layer to 553 K. Preadsorbed carbon and oxygen block the chemisorption of di-σ-bonded C2H4 and induce physisorption of C2H4 at 100 K. Preadsorbed oxygen appears to inhibit the dehydrogenation of di-σ-bonded C2H4 and induces the adsorption of π-bonded C2H4. A carbon precovered surface shows a site-blocking effect for the adsorption of C2H2.
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