Chemistry of 3-Hexyne on Ru(0001): A Reflection−Absorption Infrared Spectroscopy Study

Abstract

3-Hexyne has been adsorbed on Ru(0001) at different temperatures under ultrahigh vacuum. At 100 K and very low coverage (0.05 L), the reflection−absorption infrared (RAIR) spectrum is consistent with the formation of a surface di-σ/π complex involving the rehybridization of the acetylenic carbon atoms. This complex is oriented with the plane containing the α and β carbon atoms tilted relative to the surface. For an exposure of 0.07 L, a more compact layer of the di-σ/π species or, alternatively, a second layer of a π complex was identified by the appearance in the RAIR spectrum of the methyl symmetric modes. For higher exposures, a surface multilayer of 3-hexyne is formed, which compares well with the transmission spectrum of solid-phase 3-hexyne. The thermal decomposition of 3-hexyne on Ru(0001) follows two different paths, depending on whether the molecule is chemically bonded to the surface at low temperature (below ∼200 K) and subsequently heated or adsorbed at higher temperatures, decomposing on adsorption. In the first case, the di-σ/π complex decomposes above 143 K into an adsorbed acetylenic species and other fragments. These are mostly pumped away, but some dehydrogenates are adsorbed at 223 K as ethylidyne, identified by the small band at 1344 cm-1 assigned to the methyl umbrella mode. In the second case (adsorption at the reaction temperature) the C−C triple bond breaks completely, leaving a mixture of surface species that depends on the temperature. Upon adsorption at 243 K, only adsorbed propylidyne and ethylidyne were identified. At 273 K, the RAIR spectrum is dominated by the intense methyl umbrella mode and, in the C−H stretching region, by the methyl symmetric stretching mode of ethylidyne bonded to the surface in a 3-fold hollow site in C3v symmetry. Only at 320 K ethylidyne is completely decomposed, leaving adsorbed methylidyne as the only species identified by RAIRS.