A high-resolution electron energy loss spectroscopy study of the surface structure of benzene adsorbed on the rhodium(111) crystal face

Benzene adsorption on the Rh(111) crystal surface has been studied by HREELS, LEED, and TPD. The vibrational spectra indicate that benzene adsorbs molecularly at 300 K and is π-bonded to the surface with the ring plane parallel to the surface plane. Recent dynamic LEED calculations¹ together with the angle-dependent HREELS studies reported here establish a C_3υ(σ_d) bonding symmetry for the c(2√3 × 4)rect-C₆H₆ structure. Several other ordered benzene overlayers can be formed between 300 and 400 K depending on the benzene coverage. No large changes occur in the chemisorption bonding mode or geometry coincident with the two-dimensional ordering phase transitions in this temperature range. The vibrational spectra show that two molecular adsorption sites can be populated. Benzene adsorption is only partially reversible; less than 20% of the adsorbed benzene desorbs molecularly upon heating. The remaining benzene irreversibly decomposes, evolving hydrogen and leaving a carbon-covered surface. The TPD and HREELS data on Rh(111) and other single-crystal surfaces show correlations between the metal-benzene bond strength, the work function of the clean surface, and the frequency shifts of some of the molecular benzene vibrational modes.