THE STRUCTURE AND CHEMISTRY OF CH3 AND CH RADICALS ADSORBED ON NI(111)

A detailed analysis of the vibrational spectra of CH3, CH 2D, and CD3 adsorbed on Ni(111) and the products of their reactions is presented. The synthesis of adsorbed methyl radicals from CH 4, CH3D, or CD4 is effected by molecular beam techniques. The ability to measure these spectra by high-resolution electron energy loss spectroscopy (HREELS) at higher resolution (35 cm-1) and higher sensitivity (5×106 counts/s) has allowed new features to be observed and a symmetry analysis to be carried out. It is concluded that the CH3 radical is adsorbed with C3v symmetry on a threefold hollow site. The symmetric C-H stretch mode of CH3 and the overtone of the antisymmetric deformation mode are observed to be in Fermi resonance. At temperatures above 150 K, CH3 dissociates to form adsorbed CH. Confirmation for the assignment to a CH species is found in the observation that the spectrum measured after thermal decomposition of CH2D is a superposition of those from the decomposition of CH3 and CD 3. The adsorption site of the CH species is concluded to be a threefold hollow site and the geometry of the Ni3-C-H is concluded to be pyramidal. At temperatures above 250 K, carbon-carbon bond formation between the CH species is observed to yield C2H2. Low coverages of C2H2 are shown to dehydrogenate at 400 K. High coverages of C2H2 are shown conclusively to trimerize to form adsorbed benzene in contrast to a literature report of C2H 2 dissociation to adsorbed CH at these temperatures. The relative stabilities of the hydrocarbon species on Ni(111) are determined to be CH 3<CH+2H <1/2C2H2+2H<1/6C 6H6+H2(g). © 1995 American Institute of Physics.