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

The chemistry, induced by 50 eV electrons, of CH₃Cl 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 cm² for 1 ML CH₃Cl coverage. For coverages up to 1 ML, CH₃ and C₂H₅ radicals, some CH₄ and C₂H₆ and, perhaps, some C₂H₄ 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), -C₂H₅ and, possibly, =CHCH₃ and -CH=CH₂ fragments; an and C₂H₄ molecules. The relative concentrations of these species depend on electron fluence (EF) and CH₃Cl coverage. In TPD, CH₃Cl and C₂H₄ desorb molecularly; H atoms and hydrocarbon frragments react to produce H₂, CH₄, C₂H₄, C₂H₆, C₃H₆, C₃ H₈, C₄H₈ and C₄H₁₀ 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 CH₃Cl on clean Ag(111), the production of saturated hydrocarbons, CH₄, C₂H₆ and C₃H₈, is suppressed and the desorption of olefins shifts to lower temperatures. When D is preadsorbed, EID leads to TPD of partially deuterated, saturated hydrocarbons CH₃D, CH₂D₂, C₂H₄D₂, C₂H₅D, C₃H₅ D₃, C₃H₆D₂ and C₃H₇D. For multilayers (3-10 ML), desorption of H₂, CH₃, CH₄, C₂H₄, C₂H₅, C₂H₆, C Cl, HCl and CH₃Cl is observed during EID. The TPD products appearing below 300 K are desorption-limited HCl, C₂H₄, C₃H₆ and C₄H₈, and reaction-limited H₂, CH₄, C₂H₄, C₂H₆, C₃H₆ and C₃H₈. A strongly bound C_xH_yCl_z (x > 1) is formed during EID; it decomposes to form HCl, H₂ and surface carbon above 400 K. AgCl continues to appear at high temperatures but the amount drops as the initial multilayer CH₃Cl coverage increases.