Steric Effect in Electron−Molecule Interaction

Asscher

2009

J. Phys. Chem. A

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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 (4Φ) 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. ∆P-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 4Φ 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.

Section: Adsorbed Geometry Of Ec On O/rumentioning

confidence: 99%

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

Asscher

2009

J. Phys. Chem. A

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“…A possible origin for this difference in attachment cross section between the bare metal and the oxygen-covered one, is the 1.2 eV higher work function of the O/Ru(001) compared to clean Ru(001). 23 The lower energy position of the Fermi level relative to the vacuum level in the O/Ru substrate, leads to excitation by the 6.4 eV photons to energy range that apparently does not overlap well the relevant excitation level of the caged EC molecule. We conclude that as a result, the probability for electron attachment in the O/Ru sample is about an order of magnitude smaller than that on the clean ruthenium sample, resulting in the overall smaller decomposition rate of the parent molecule: s Ru E 20 s O/Ru .…”

Section: Photochemistry Of Caged Ecmentioning

confidence: 99%

Photochemistry of ethyl chloride caged in amorphous solid water

Phys. Chem. Chem. Phys.

Asscher

2008

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Caging and photo-induced decomposition of ethyl chloride molecules (EC) within a layer of amorphous solid water (ASW) on top of clean and oxygen-covered Ru(001) under ultra-high vacuum (UHV) conditions are presented. The caged molecules were estimated to reside 1.5 +/- 0.2 nm above the solid surface, based on parent molecule thermal decomposition on the clean ruthenium. Dissociative electron attachment (DEA) of the caged molecules following 193 nm laser irradiation, result in initial fragmentation to ethyl radical and chloride anion. It was found that photoreactivity on top of the clean ruthenium surface (Ru) is twenty times faster than on the oxygen-covered surface (O/Ru), with DEA cross sections: sigma(Ru) = (3.8 +/- 1) x 10(-19) cm(2) and sigma(O/Ru) = (2.1 +/- 0.3) x 10(-20) cm(2). This difference is attributed to the higher work function of oxygen-covered ruthenium, leading to smaller electron attachment probability due to mismatch of the ruthenium photo-electron energy with the adsorbed EC excited electron affinity levels. EC molecules fragmented within the cage, result in post-irradiation TPD spectra that reveal primarily C(4)H(8), C(3)H(5) and C(3)H(3), without any oxygen-containing molecules. Unique stabilization of the photoproducts has been observed with the first layer of water molecules in direct contact with the substrate, desorbing near 180 K, a significantly higher temperature than the desorption of fully caged molecules. This study may contribute for understanding stratospheric photochemistry and processes in interstellar space.

“…The photochemical consequences that result in the observation of steric effect in electron-molecule interaction were presented. 41 In this section we shall present another type of neighbor or environment effect on the organization and chemistry of molecules on surfaces. Here, the confinement of CO 2 molecules within a matrix of ice will be described, together with the resulting photo-excitation of such system.…”

Section: Photochemistry Of Caged Molecules: Co 2 @Icementioning

confidence: 99%

Photochemistry of molecules at confined environment: CD₃Br/O/Ru(001) and CO₂@Ice

Berger

Lilach

Israel Journal of Chemistry

et al. 2005

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The photochemistry of molecules constrained within a confining environment on surfaces has been studied. Orientation of methyl bromide could be controlled methyl down or up by varying the pre‐adsorbed oxygen coverage due to electrostatic interactions on Ru(001) under UHV conditions. Irradiation of the coadsorption system at 193 nm has shown that the resulting photochemical activity is sensitive to the molecular orientation. Photodesorption and dissociation cross sections were 1.0·10−19 cm−2 for methyl‐down and 3.0·10−19 cm−2 for the methyl‐up configurations. This observation represents the first report of the steric effect in electron‐molecule interaction due to the dissociative electron attachment mechanism of photochemical processes on surfaces. A second system of CO2 molecules caged within ice has also been studied. Here the trapped carbon dioxide molecules cannot leave the surface at their normal desorption temperature near 100 K, but are explosively desorbing at the onset of ice evaporation near 165 K. Upon UV irradiation, enhanced dissociation to adsorbed CO and oxygen is recorded. In addition, a new reactivity channel is observed to form H2CO, tentatively identified as formaldehyde. The relevance of photochemistry of caged molecules within ice to interstellar hydrocarbon formation as a possible route for the origin of life is discussed.