GRAZING-INCIDENCE X-RAY-DIFFRACTION ON CS GE(001)(2X1) - EVIDENCE FORCAVE SITE ADSORPTION/

Grazing incidence X-ray diffraction has been used to investigate the a tomic structure of Cs adsorbed on Ge(001)(2x1) at room temperature. Fo r about half monolayer coverage (1 monolayer=6.25 x10(14) atoms/cm(2) for Ge(001)) the analysis of the data set consisting of 12 symmetry in dependent in plane (Q(z)=0) reflections and of four fractional order r eciprocal lattice rods (62 independent reflections in total) gave clea r 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 a re found to be strongly disordered along the surface normal. Within a static disorder model this is accounted for by splitting the Cs leadin g to different height levels, d(perpendicular to 1)=1.73(11) Angstrom and d(perpendicular to 2)=0.44(11) Angstrom above the Ge dimer level. Minimum Cs-Ge neighbor distances of R = 3.14(21)Angstrom and 3.50(16) Angstrom for the split atoms to nearest Ge atoms might indicate a mixe d strength of interaction between adsorbate and substrate. A dynamical model allowing an unsplitted Cs atom to strongly vibrate along the su rface normal by a root mean square amplitude of [u(33)(2)](1/2)=0.93 A ngstrom leads to an equilibrium distance of R=3.56(20) Angstrom to sec ond layer Ge also suggesting some charge transfer to the substrate. Wh ereas the Ge dimer bonding length is found not to be strongly affected by Cs adsorption (R(D)=2.45(11) Angstrom), extensive relaxations are observed down to the 3rd substrate layer.