Interaction of Ethyl Chloride with Amorphous Solid Water Thin Film on Ru(001) and O/Ru(001) Surfaces

Abstract

The adsorption of ethyl chloride (EC) on clean and oxygen covered Ru(001) surfaces and its interaction with coadsorbed amorphous solid water (ASW) is reported based on temperature-programmed desorption (TPD) and work function change (DeltaPhi) measurements. Adsorption of EC is characterized by a decrease of 2.1 eV in work function at monolayer coverage. Flipped adsorption geometry on average (ethyl facing the surface) is found in the second layer with a 0.3 eV increase in the work function. On ruthenium substrates the orientation of EC molecules could be controlled chlorine down or up by varying the oxygen coverage, as indicated by work function change measurements. DeltaP-TPD from clean Ru(001) surface reveals a complete dissociation of the EC molecules on the clean Ru(001) surface at coverages up to 0.1 +/- 0.05 ML. At higher coverage it leads to two distinct TPD peaks: at T =175 and 120-130 K, for the monolayer and multilayer coverage, respectively. Compression of preadsorbed 0.3 ML EC molecules into small islands on the surface occurs when ASW layers at coverage in the range 0.5-6 bilayers (BL) are adsorbed on top of the EC covered surface, associated with a shift in peak desorption from 190 K down to 125 K. A Caging process of EC molecules within the layers of ASW takes place by adsorbing more than 6 BL until a fully caged system has developed above 20 ASW BL. A reversed TPD peak shift of the trapped EC molecules occurs from its compressed phase at 125 K up to 165 K, the onset for ASW transition to crystalline ice and desorption. The detachment process leads to partial flipping over the geometry of the EC at the second layer, as determined by DeltaPhi measurement. The distance of the lifted and caged EC molecules from the Ru surface has been determined to reside 1.0 +/- 0.2 nm above the solid surface, based on the thermal dissociation and hydrogen uptake measurements of sandwiched EC molecules between a variable thickness ASW film and a fixed 10 BL layer, on top of clean Ru(001). We conclude that the caged EC molecules are trapped in a micelle-like geometry with the ethyl groups pointing inward. This conclusion is based on photodecomposition studies of the caged EC molecules at 193 nm, as a function of EC initial coverage.