Grazing incidence X-ray diffraction on Cs/Ge(001)(2 × 1): evidence for cave site adsorption

Grazing incidence X-ray diffraction has been used to investigate the atomic structure of Cs adsorbed on Ge(001)(2 × 1) at room temperature. For about half monolayer coverage (1 monolayer = 6.25 × 1014atomscm2 for Ge(001)) the analysis of the data set consisting of 12 symmetry independent in plane (QZ = 0) reflections and of four fractional order reciprocal lattice rods (62 independent reflections in total) gave clear evidence for the occupation of the cave site which is in the groove between the dimer rows and above the 3rd layer Ge-atom. The Cs atoms are found to be strongly disordered along the surface normal. Within a static disorder model this is accounted for by splitting the Cs leading to different height levels, d⊥ 1 = 1.73(11) A and d⊥ 2 = 0.44(11) A above the Ge dimer level. Minimum Cs-Ge neighbor distances of R = 3.14(21) A and 3.50(16) A for the split atoms to nearest Ge atoms might indicate a mixed strength of interaction between adsorbate and substrate. A dynamical model allowing an unsplitted Cs atom to strongly vibrate along the surface normal by a root mean square amplitude of 〈u332〉12 = 0.93A leads to an equilibrium distance of R = 3.56(20) A to second layer Ge also suggesting some charge transfer to the substrate. Whereas the Ge dimer bonding length is found not to be strongly affected by Cs adsorption (RD = 2.45(11) A), extensive relaxations are observed down to the 3rd substrate layer.