Thermal evolution of acetylene overlayers on Pd(111)

The thermal evolution of acetylene adsorbed on the (111) surface of palladium has been studied by high resolution electron energy loss spectroscopy (HREELS). This study encompasses the temperature range 120-800 K for acetylene exposures of 0.25-3.5 langmuirs and includes HREELS data for both specular and off-specular scattering. The spectra are interpreted in terms of acetylene evolution to ethylidyne via a vinylidene intermediate, with the additional hydrogen supplied by the residual gas. Evidence for benzene formation is also seen in the appearance of the benzene upsilon (4) (720 cm(-1)) band. Subsequent heating to 400 K results in dehydrogenation of ethylidyne to CCH species. A spectral normalization method is employed to compare intensities of vibrational bands as a function of temperature and proves useful in analysis of the temperature dependence of the surface species. We find that 0.25-langmuir exposure results in an essentially pure ethylidyne layer whereas higher coverages eventually result in lower ethylidyne production, likely due to steric hindrances on the vinylidene intermediate and the competing channel of benzene production.