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 DELTA-phi. Electron induced dissociation (EID) occurs very efficiently with an initial total cross section of, at most, (5.5 +/- 0.5) x 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; 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 fragments react to produce H-2, CH4, C2H4, C2H6, C3H6, C3H8, 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, C3H5D3, C3H6D2 and C3H7D. For multilayers (3-10 ML), desorption of H-2, CH3, CH4, C2H4, C2H5, C2H6, 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 H-2, CH4, C2H4, C2H6, C3H6 and C3H8. 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 CH3Cl coverage increases.