The surface chemistry of methyl and methylene radicals adsorbed on Cu(111)

The interactions of methyl and methylene radicals on Cu(1 1 1) were investigated with XPS, AES and HREELS under various exposure conditions. The CH2 and CH3 radicals are generated through a hot nozzle source with ketene and azomethane gases. It is shown that with substrate at 300 K, the impinging CH3 radicals are trapped mainly as CH3(ads), while a part of the adsorbate decomposes to form CH2(ads) and H(ads). H atoms are found to desorb at about 380 K, while the chemisorbed hydrocarbon adspecies desorb at about 420 K. In drastic contrast, exposing the clean Cu surface to methylene radicals results nor only in the trapping of CH2(ads), but also in the formation of complex aromatic species. The adlayer is sensitive to annealing at elevated temperatures, Desorption and partial conversion to methylidyne take place at around 420 K. The CH(ads) species can survive up to 700 K and then decomposes to form residual carbon above 800 K, In both radical-Cu(lll) systems, surface coverage appears to saturate near one monolayer. The relative concentrations of different surface species in the adlayer, however, depend on the amount of radical exposure. The reaction properties of the two systems are compared and discussed. (C) 1999 Elsevier Science B.V. All rights reserved.