ADSORPTION OF ACETYLENE AND HYDROGEN ON PD(111) - FORMATION OF A WELL-ORDERED ETHYLIDYNE OVERLAYER

The C2H2(+ H)/Pd(111) system has been studied using high resolution co re level spectroscopy and low energy by electron diffraction (LEED). T wo ordered phases were found when adsorbing C2H2 at 125 K, giving rise to a (2 x 2) and a (root 3 x root 3)R30 degrees LEED pattern, respect ively. When preadsorbing H at 110-150 K, the sticking probability of t he C2H2 molecules is strongly reduced and only the (root 3 x root 3)R3 0 degrees overlayer is formed. In all three of the ordered structures the C2H2 molecules are suggested to occupy the hollow sites. Heating o f the (root 3 x root 3)R30 degrees C2H2+H layer to 350 K yields a well ordered (root 3 x root 3)R30 degrees overlayer consisting exclusively of ethylidyne (CCH,) positioned in the hollow sites. The formation of ethylidyne is furthermore found to be preceded by the formation of an intermediate species (''beta'') which probably is vinylidene. (C) 199 8 Elsevier Science B.V. All rights reserved.