Geometrical structure of benzene absorbed on Si(001)

Although many details are known from experiments about the adsorption of benzene on Si(001), no definite structural proposal has so far been derived. Thus, "first principles" density functional energy optimizations have been performed for various structure models which were chosen in accordance with available experimental information on C6H6/Si(001). By means of symmetry-resolved projected densities of states, the photoemission spectrum of benzene on Si(001) is simulated and analyzed according to dipole selection rules for each of the structure models. In addition, the vibrational properties of the various adsorption complexes are calculated using cluster models and the character of the vibrational modes is determined. The various structure models exhibit several distinct differences in electronic and vibrational properties. By comparison to experimental photoemission and energy-loss spectra of C6H6/Si(001), the structure of the adsorption complex is derived, a C-2v symmetric 1,4-cyclohexadiene-like adsorption complex with a flat-lying benzene molecule, di-sigma bound to one silicon surface dimer, This is consistent with the results of "first principles" geometry optimizations. Furthermore, based on this structure model, a rationalization of the unexpected (2 x 1) electron diffraction pattern observed for C-6/H-6/Si(001) at a saturation coverage of 1/4 ML is given. (C) 1998 Elsevier Science B.V. All rights reserved.