CH3I and C2H5I on Au(100): adsorption and reaction

Abstract

The adsorption and reaction of CH3I and C2H5I on a Au(100) surface have been studied by a combination of temperature-programmed desorption/reaction (TPD/R), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), reflection-absorption infrared spectroscopy (RAIRS), high-resolution electron energy loss spectroscopy (HREELS) and work function change (Δф) measurements. Both alkyl halides adsorb and desorb mainly in molecular form without decomposition. Only 4% of the first layer CH3I molecules dissociate on the surface; the methyl groups formed by this reaction couple to form ethane at ∼ 360 K. By contrast, 20% of the first layer C2H5I molecules decompose and the final surface reaction products are ethylene, ethane and butane. The dominant product is C4H10 which is formed at ∼ 270 K. Repulsive adsorbate-adsorbate interactions are observed in the first adsorption layer for both alkyl iodides. For CH3I (but not for C2H5I), a high coverage metastable adsorption state characterized by a desorption temperature lower than the multilayer desorption temperature is observed. The structure of the metastable state has yet to be determined, but the results suggest the presence of a CH3I bilayer in which the CH3I molecules in the first layer are oriented with their CI bonds approximately parallel to the surface and the molecules in the second, metastable layer have a tilted orientation similar to the average orientation in the multilayer. To the best of our knowledge, this is the first observation of a metastable adsorption state for alkyl halides on metal surfaces.