DENSITY-FUNCTIONAL INVESTIGATION OF THE GEOMETRIC AND ELECTRONIC-STRUCTURE OF ETHYLENE ADSORBED ON SI(001)

A detailed first-principles density functional analysis of the geometr ic and electronic properties of ethylene adsorbed on the dimer reconst ructed Si(001)-(2 x 1) surface is presented. This theoretical study wa s carried out in close reference to a recent angle-resolved photoemiss ion spectroscopy investigation of the same adsorption system. Adsorbat e weighted Kohn-Sham one-particle spectra are calculated and compared to the band structure derived from the angle-resolved photoemission sp ectra. In addition, the symmetry character of the concomitant Bloch wa ves is determined to yield information which can directly be related t o the results of a dipole selection rule analysis of the corresponding photoemission signals. Total energy minimization of a model slab reve als a distortion of the adsorption complex at saturation coverage to l ocal C-2 symmetry involving an 11 degrees rotation of the ethylene mol ecule around the surface normal and a 27 degrees twist of the methylen e groups around the C-C axis. This finding is confirmed by a compariso n of the calculated band dispersions with those found in the angle-res olved ultraviolet photoelectron spectroscopy (ARUPS) experiments. The driving forces for the distortion of the adsorption complex can be tra ced to direct Pauli repulsion between the hydrogen atoms of neighborin g ethylene molecules and to a bonding overlap contribution from the et hylene 1b(2g)-derived orbitals of the adlayer. (C) 1998 American Insti tute of Physics.