Benzene multilayers: A model for their anisotropic growth from vibrational spectroscopy and thermal desorption

Multilayers of benzene grown on Ru(001) at about 120 K were investigated by temperature programmed desorption (TPD) and high resolution electron energy loss measurements (HREELS). In agreement with earlier work, three TPD-states are observed which grow in with increasing doses in the order α₁ - α₂ - α₃, where α₁ desorbs at the highest and α₂ at the lowest temperature, and the latter is quenched when α₃ is formed in between. In disagreement with earlier interpretations, these are identified as the first physisorbed layer (α₁) on top of the chemisorbed layer, a metastable transition layer (α₂), and "bulk" multilayers (α₃). An intensity analysis of in-plane and out-of-plane vibrational modes observed by HREELS in the dipole scattering mode shows that the benzene molecules in the α₁-layer are oriented essentially parallel to the surface (as is the chemisorbed layer), while in the metastable α₂-layer strongly tilted molecules predominate. The molecules in the higher "bulk" layers possess a lower mean tilt angle. The α₁-layer has a distinctly higher binding energy than the subsequent ones, and this is assigned to residual interaction with the substrate. The lower desorption temperature of α₂ compared to the "bulk" α₃ benzene may be due to an energy gain of α₃ by correlated tilts to produce a netted structure, compared to a parallel packing of approximately perpendicular molecules in the α₂-layer. If the total coverage of physisorbed molecules exceeds 2.3 monolayers, slow annealing leads to an irreversible transition of α₂ to α₃, corroborating the interpretation of α₂ as metastable. Experiments using isotopic layering suggest that the geometry of the α₁ -layer is unchanged by additional layers, but that exchange of molecules between α₁ and α₂ takes place already upon adsorption at 115 K. The results are used to derive a model of the multilayer adsorption and desorption.