Interface bonding of bithiophene on clean and Cs-covered Al(111) surfaces

The vibrational structure of bithiophene adsorbed on clean and a number of Cs-covered Al(111) surfaces was investigated using high-resolution electron energy loss spectroscopy (HREELS). Al(111) surfaces doped with Cs were pre pared as a function of coverage and temperature to vary the capacities and geometries of surface donors for bithiophene adsorption. On clean Al(111) H REELS analysis indicates an adsorption geometry with both thiophene units p arallel to the metal surface, suggesting an eta(5)-type multicentre bonding configuration. On the Cs-covered Al(111) surfaces Cs-bithiophene charge tr ansfer is important, when the Cs adatoms are metallic. Comparison with HREE LS spectra of Cs-doped condensed bithiophene thin films indicates that at m ost one electronic charge may be transferred to the adsorbed molecule, yiel ding Cs+-bithiophene(-) surface complexes. The amount of charge transfer de pends on the donor capability of the Cs adatoms, i.e. positive ionic Versus metallic as a function of coverage, and on the Al-Cs surface geometry. In general, tilted surface orientations are proposed for bithiophene on the Cs -covered surfaces, with several bonding mechanisms coexisting at intermedia te Cs coverages. The Cs-covered Al(111) surfaces, however, display poor spe cular scattering properties, thus limiting the capabilities of HREELS to de termine molecular surface orientation via the dipolar surface selection rul e. (C) 1999 Elsevier Science B.V. All rights reserved.