Abstract:
The orientation of the endohedral La atom in La@C82 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 70.5° off-normal (111) Bragg reflections. The La (111) profiles of the La@C82
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 (60.77) Å 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.
