Symmetry and electronic structure of benzene adsorbed on single-domain Ge(100)-(2x1) and Ge/Si(100)-(2 x 1)

The electronic structure of benzene adsorbed on single-domain Ge(100)-(2x1) and on a single.-domain Ge monolayer grown on Si(100)-(2x1) has been inves tigated by angle-resolved ultraviolet photoemission spectroscopy (ARUPS) us ing linearly polarized synchrotron radiation and by temperature-programmed desorption (TPD) spectroscopy. For benzene chemisorption on both surfaces; C-2 upsilon adsorption symmetry is found on the basis of the ARUPS data. Th e detailed analysis indicates a flat-lying molecule which is di-sigma -bond ed to a Ge-Ge dimer and which exhibits a 1,4-cyclohexadiene-like electronic structure. The similarity of the ARUP spectra for benzene on Ge(100), on G e/Si(100), and on Si(100) indicates nearly identical electronic structures. Benzene TPD spectra on a Ge monolayer on Si(100) show two desorption peaks at 311 and 369 K which are interpreted as desorption from terrace and step sites, respectively. Similarly, benzene desorption on Ge(100) leads to des orption peaks at about 230 and 250 K, again interpreted as desorption of ch emisorbed benzene from terrace and step sites. Additionally, a weakly bound benzene species is found on Ge(100) which desorbs between 155 and 220 K an d which is absent on the other surfaces. ARUPS data indicate C-1 adsorption symmetry and an electronic structure similar to that of gas-phase benzene for this species.