A model of ethylene and acetylene adsorption on the (111) surfaces of platinum and nickel

Despite the application of a variety of surface sensitive techniques to the adsorption of simple hydrocarbons on well characterized metallic surfaces, no consistent picture has appeared. We review briefly the published spectroscopic results of ultraviolet photoelectron spectroscopy (UPS) and electron energy loss spectroscopy (EELS) which probe, respectively, the electronic and vibrational structure of the surface-molecular complex, and we consider appropriate free molecular analogues, not only in their ground state but also in their first excited states. A simplified approach to determine the chemisorption geometry from UPS level shifts and EELS is presented. The technique allows an isolation of distortion induced shifts from the total relaxation shift, and we find that the true relaxation shift is rather constant, approximately 2.1 eV for the cases considered. These shifts can then be used to estimate the distance of the molecule to the surface. We concentrate primarily on four systems, C₂H₂ and C₂H₄ on Ni(111) and Pt(111), adsorbed at low temperature (below the onset of dissociation). Depending on the metal, the hydrocarbon can adsorb in a di-σ arrangement or with a distortion resembling the lowest energy configuration of the first excited state of the free molecule. We also consider briefly C₂H₄ on Ag and Cu in which no distortion occurs. The distortions that resemble the first excited states might occur as a consequence of donation of bonding (backbonding) electrons from (to) the normally filled π (empty π¹) to (from) the empty (filled) d-band states of the metal. The net effect on the hydrocarbon to partially empty the π level and fill the π¹ level, is analogous to a low excitation of the free molecule, π → π¹. For C₂H₄ (planar in the ground state), the lowest excitation is the triplet T-state (3–4 eV) of minimal energy for a 90° twisted configuration with a lengthened C-C bond. Acetylene is a linear molecule in the ground state, but cis- or trans-bent for the triplet excitations, ~a (5.2 eV) or ~b (6.0 eV), respectively. Chemisorbed geometries derived from these configurations seem possible for C₂H₄ on Ni(111) and C₂H₂ on Pt(111), while interchanging the adsorbates and substrates gives di-σ bonding, (sp³ hybridization), as proposed previously in the literature. For C₂H₄ on Ni(111), two of the hydrogens are twisted into the surface which leads to a softening of the CH vibrational frequency. For the four systems considered, the data are consistent with the C-C bond essentially parallel to the surface, but tilted orientations are not ruled out. While the models are clearly oversimplified, they suggest an interesting point of departure for likely chemisorption geometries. Also, some intriguing correspondences to the (presumed) location of the normally empty π¹ level and the d-band are noted.