STRUCTURAL-ANALYSIS OF A SHORT-RANGE ORDERED LAYER WITH SEVERAL ADSORPTION SITES - O NI(111)/

A detailed structural analysis of O/Ni(111) around a coverage of 0.3 M L is presented, where a disordered phase is formed in thermal equilibr ium. The analysis of LEED- and DLEED-I(E) curves shows that both hcp a nd fee sites are occupied simultaneously at these coverages, whereas o xygen exclusively adsorbs on fee sites in the ordered p(2 x 2) and (ro ot 3 x root 3)R30 degrees structures, which are completed at coverages 0.25 and 0.33, respectively. This information, and the fraction of hc p and fee occupation as a function of coverage, can already be obtaine d from an analysis of the integral order spots, as shown in our analys is. Integrated intensities around the (1/2,0) and (3/2,0) positions ha ve been evaluated in addition. In order to determine the short range o rder correlations between the adsorbed atoms, which govern the intensi ty distribution in reciprocal space, the complex scattering amplitudes , especially the phases, are needed. They were determined for selected K-points from full dynamical scattering calculations of ordered struc tures with fee site and hcp site occupation. It turned out that a succ essful kinematic simulation of the LEED patterns could be carried out only for energies where the phase difference between hcp and fee sites was small at all K-points of the p(2 x 2) structure. At these energie s, the experimentally observed complex angular distribution of diffrac ted low energy electrons could be reproduced in all details with a mod el of small p(2 x 2) domains, with oxygen in fee sites separated by a random distribution of domain walls (wall concentration fixed by cover age) in which the hcp sites are occupied. For other energies, the fast variation of these phase differences in reciprocal space highly compl icates the situation and makes simulations unreliable.