GEOMETRIC AND ELECTRONIC-STRUCTURE OF POTASSIUM ON RH(111)

The interaction of potassium with Rh(111) was studied by AES, REELS, LEED, TDS, UPS and work function (WF) measurements. At low coverages (THETA(K) < 0.1) PHI decreases linearly followed by a shallow minimum at THETA(K) almost-equal-to 0.2 and a slight reincrease. Three different LEED K-superstructures at THETA(K) = 0.25 (2 x 2), THETA(K) = 0.33 (square-root 3 x square-root 3 R30-degrees) and at THETA(K) = 0.5 (2 x 2) have been observed corresponding to a nearest K-K neighbour distance for the first two structures of 5.38 angstrom and 4.66 angstrom, respectively. The square-root 3 x square-root 3 R30-degrees structure is compressed by 6% compared to the surface density of (I 10) K planes. The multilayer desorption in TDS is observed only at K-coverages beyond 0.5 which is generally correlated with a metallic (potassium like) character of the overlayer. In the K-coverage range 0-0.33 the electronic binding energy of the K(3p) level shifts to lower binding energy (I 8.4-17.8eV). Above THETA(K) = 0.5 a new K(3p) peak develops at the same energetic position as it is observed for a thick potassium film (18.6eV). The transition of the electronic properties of the K-overlayer from an ionic (low coverage) to a covalent (high coverage) bonding character is also reflected in the electron loss experiment. Only above THETA(K) = 0.5 the plasmon loss characteristic of thick metallic K at 2.5eV can be detected, even though already around 0-33 the K-K nearest neighbour distance corresponds to the covalent value.