Interaction of Alkanethiols with Single Crystal Iron: The Low-Temperature Decomposition of Ethanethiol on the Fe(100) Surface

The decomposition mechanism of ethanethiol (C₂H₅SH) on the Fe(100) surface under ultrahigh vacuum has been investigated using temperature programmed reaction spectroscopy (TPRS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (HREELS). Upon adsorption at 100 K, ethanethiol undergoes S‒H bond scission to form a surface ethanethiolate (‒SC₂H₅). The ethanethiolate species starts to decompose below 253 K via C‒S bond cleavage which is identified as the rate-determining step. HREELS data suggest different mechanisms for the ethanethiolate decomposition at different coverages. On the unsaturated surface, the C‒S bond cleavage is followed by β-hydrogen elimination leading to the formation of ethylene and surface hydrogen. Part of the ethylene molecules interact with reactive iron sites and further decompose to surface CH, CCH, and surface carbon, while the rest of the ethylene evolves to the gas phase. For the saturated surface, ethane is also observed while ethylene remains the major reaction product. Further decomposition of the hydrocarbons on the surface was prohibited by the passivation effect of the coadsorbed species. The decomposition of the saturated ethanethiolate overlayer leaves a c(2×2) sulfur overlayer on the Fe(100) surface.