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Low energy electron induced chemistry: CH3Cl on Ag(111)

Surface Science
Publication Date
DOI: 10.1016/0039-6028(92)90906-m
  • Chemistry


Abstract The chemistry, induced by 50 eV electrons, of CH 3Cl on clean, Cl-covered and D-covered Ag(111) at 100 K has been studied using TPD, XPS, UPS and Δφ. Electron induced dissociation (EID) occurs very efficiently with an initial total cross section of, at most, (5.5±0.5)×10 −16 cm 2 for 1 ML CH 3Cl coverage. For coverages up to 1 ML, CH 3 and C 2H 5 radicals, some CH 4 and C 2H 6 and, perhaps, some C 2H 4 molecules desorb during EID; very few Cl-containing species desorb. At the surface, the following are synthesized and retained: H and Cl atoms; CH x ( x = 2,3), −C 2H 5 and, possibly, =CHCH 3 and −CH=CH 2 fragments; an and C 2H 4 molecules. The relative concentrations of these species depend on electron fluence (EF) and CH 3Cl coverage. In TPD, CH 3Cl and C 2H 4 desorb molecularly; H atoms and hydrocarbon frragments react to produce H 2, CH 4, C 2H 4, C 2H 6, C 3H 6, C 3 H 8, C 4H 8 and C 4H 10 below 300 K; the only higher temperature product is AgCl, which desorbs at 750 K. No dehydrogenation occurs during TPD. Preadsorbed Cl has a dramatic influence; compared to 1 ML CH 3Cl on clean Ag(111), the production of saturated hydrocarbons, CH 4, C 2H 6 and C 3H 8, is suppressed and the desorption of olefins shifts to lower temperatures. When D is preadsorbed, EID leads to TPD of partially deuterated, saturated hydrocarbons CH 3D, CH 2D 2, C 2H 4D 2, C 2H 5D, C 3H 5 D 3, C 3H 6D 2 and C 3H 7D. For multilayers (3–10 ML), desorption of H 2, CH 3, CH 4, C 2H 4, C 2H 5, C 2H 6, C Cl, HCl and CH 3Cl is observed during EID. The TPD products appearing below 300 K are desorption-limited HCl, C 2H 4, C 3H 6 and C 4H 8, and reaction-limited H 2, CH 4, C 2H 4, C 2H 6, C 3H 6 and C 3H 8. A strongly bound C x H y Cl z ( x > 1) is formed during EID; it decomposes to form HCl, H 2 and surface carbon above 400 K. AgCl continues to appear at high temperatures but the amount drops as the initial multilayer CH 3Cl coverage increases.

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