X-RAY STRUCTURE-ANALYSIS ON ALKALI-METALS ADSORBED ON GE(001)(2X1)

Surface x-ray diffraction has been used to investigate the geometric s tructure of Cs, K, and Na adsorbed on Ge(001)(2 x 1) at room temperatu re. At low and saturation coverage corresponding to about 0.3 monolaye r (ML) and 0.6-0.7 ML, respectively (1 ML = 6.25 x 10(14) atoms cm(-2) ), adsorption takes place in the large grooves between the Ge-dimer at oms. For all adsorption systems investigated, the statistical occupati on of two different sites is observed: first, above the third-layer Ge atoms (T3), and second, in an asymmetric site close to the dangling b onds of the Ge-dimer atoms (a-T4). At about half saturation coverage w e have evidence for the preferential occupation of the adsorption site T3 indicating a coverage-dependent ordering and the formation of a li near densely packed chain along the [110] direction of the substrate. Whereas the bond lengths of the T3-adsorbed alkali metals to nearest-n eighbor Ge atoms are in the regime found for bulk alkali-metal-ae stru ctures, which can be interpreted as indicative of covalent bonding, th ey are shorter for alkali metals adsorbed in a-T4 to the Ge-dimer atom s, where average distances in the regime between covalent and ionic bo nding are observed. This can be interpreted by partial charge transfer to the empty dangling bond states. The alkali metals Cs and K are gen erally found to occupy different adsorption heights, which can be rela ted to a strong disorder normal to the surface due to steric reasons. This is not observed for the smaller Na atoms. Generally, the Ge-dimer bond length (2.45 Angstrom) and the dimer asymmetry (inclination angl e of the dimer bond to the surface +/- 17.6 degrees) are not influence d by alkali-metal adsorption within 0.1 Angstrom and about 4 degrees, respectively. Shifts of Ge atoms at least down to the fourth layer bel ow the surface are observed.