Adsorption and Decomposition of 1-Alkane thiols on the Fe(100) Surface

The adsorption and decomposition of 1-alkanethiol molecules (C_nH_2n+SH, n= 4, 6,10) on the Fe(100) surface under ultrahigh vacuum have been investigated using temperatureprogrammed 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, 1-alkanethiol molecules undergo S-H bond scission to form a surface alkanethiolate (-SC_nH_2n+1). The alkanethiolate starts to decompose below 255 K via C-S bond cleavage which is identified as the rate-determining step. HREELS data suggest different mechanisms for the alkanethiolate decomposition at different coverages. On the unsaturated surface, the C-S bond cleavage is followed by β-hydrogen elimination leading to the formation of corresponding terminal alkene (C_nH_2n) and surface hydrogen. Part of the alkene molecules interact with reactive iron sites and further decompose to surface hydrocarbon species and surface carbon, while the rest of the alkene desorbs into the gas phase. For the thiol-saturated surface, corresponding alkane (C_nH_2n+2) is also observed in the gas phase. At this coverage, further decomposition of the hydrocarbons on the surface is prohibited by the passivation effect of the coadsorbed species. At saturation coverage, the decomposition of alkanethiolate overlayers leave c(2x2) sulfur overlayers corresponding to 0.5 monolayer of sulfur on the Fe(100) surface. The decomposition of all the alkanethiol molecules studied here occurs at the same temperature, indicating that the reactivity of the metal substrate is critical to the bonding interactions at the interface.