SYNTHESIS AND SPECTROSCOPIC IDENTIFICATION OF ETHYLIDYNE ADSORBED ON NI(111)

The interaction of ethylene adsorbed on Ni(111) with gas-phase H atoms has been investigated. The major adsorbed reaction product is identif ied by high-resolution electron energy loss spectroscopy to be ethylid yne (C-CH3). This study is the first direct spectroscopic observation of a C-CH3 species adsorbed on Ni in an ultrahigh-vacuum environment. Spectra of four isotopomers, C-CH3, C-13-(CH3)-C-13, C-CD3, and C-13-( CD3)-C-13, are reported, and a complete and consistent vibrational ass ignment of their fundamental modes is presented. Based on this assignm ent, a force field is derived from the measured vibrational frequencie s using a normal-modes analysis and is found to be in good agreement w ith that deduced from IR spectra of an ethylidyne species in an organo metallic complex. Inspection of the eigenvectors of the normal-mode di splacements reveals that substantial mixing of harmonic bond motions i s the origin of the unusual upshift in frequency of the C-C stretching mode upon deuteration. A quantitative determination of the relative d ynamic bond dipole moments demonstrates that the changes in intensity and dipole activity of the C-C stretching and symmetric CH3 deformatio n modes upon deuteration, phenomena common to all C-CD3 spectra, also arise from extensive mixing of bond motions. A detailed analysis of th e spectra strongly suggests a C-3 upsilon or C-3 local environment for ethylidyne and a 3-fold hollow adsorption site.