Orientation and constraints of endohedral lanthanum in La@C82 molecules adsorbed on Cu(111) -: art. no. 045424

The orientation of the endohedral La atom in La@C-82 adsorbed on Cu(111) has been determined using the normal-incidence x-ray standing wave (NIXSW) technique. The NIXSW measurements were carried out using both the normal [111] and 70.5degrees off-normal [(1) over bar 11] Bragg reflections. The La[(1) over bar 11] profiles of the La@ C-82 monolayer and submonolayers are found to be similar to the [111] profile of the multilayer, while the [111] profiles of the monolayer and submonolayers are distinctly different with much lower peak height. This indicates significant ordering of the La atoms in the monolayers with respect to height above the Cu surface but little lateral ordering. The results of curve fits to the monolayer profiles show that the La atoms are regionally located at 1.62 (+/-0.77) Angstrom above an extended scattering plane from the crystal. Cooling the monolayers to a temperature of 170 K increases ordering of the La atoms with respect to height above the surface. The NIXSW results demonstrate that the endohedral molecules adopt a restricted orientation upon adsorbing on the surface and the La atom resides on either the upper or lower half of the cage. Theoretical calculations show that the most stable La positions are located in the top half of the fullerene cage, which supports the La upper configuration. The results are explained in terms of electrostatic interactions between the endohedral atom, the fullerene cage, and the substrate.