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Schlagwörter:
Low energy electron diffraction (LEED); Photoelectron spectroscopy; Thermal desorption spectroscopy; Sticking; Surface chemical reaction; Chlorine; Silver; Alkenes
Zusammenfassung:
The adsorption of CCl4 on Ag(1 1 1) has been investigated from 100 K to 300 K using absolute sticking probability measurement, temperature programmed desorption, Auger electron spectroscopy, low energy electron diffraction, ultra-violet photoelectron spectroscopy and X-ray photoelectron spectroscopy. At 100 K, CCl4 adsorbs molecularly with a sticking probability of 1.0, forming a (3 × 3) adsorption structure. At 300 K the following overall reaction occurs, 2CCl4(g)→4Cl(chem)+C2Cl4(g), with an initial sticking probability of S0 = 0.25, indicating that activated dissociative adsorption is the initial step in the above reaction. The saturated surface at 300 K consists entirely of chemisorbed chlorine which undergoes an order/disorder transition to a sharp (√3 × √3)R30° structure on cooling below 180 K. Temperature programmed desorption of CCl4 adsorbed at 100 K shows a multilayer peak at 140 K and a first layer peak at 203 K, where part of the molecularly adsorbed CCl4 desorbs and the rest is converted to a surface carbene, :CCl2, and chemisorbed chlorine. At 248 K the adsorbed carbenes react to form C2Cl4 which promptly desorbs, as it is formed above its normal desorption temperature of 210 K.
