A SCANNED-ENERGY MODE PHOTOELECTRON DIFFRACTION STUDY OF THE STRUCTURE OF NI(111)(2X2)-O

A scanned-energy mode photoelectron diffraction study has been conduct ed of the Ni(111)(2 x 2)-O chemisorption phase using O Is modulation s pectra recorded in a range of different emission directions. The O ato m is found to occupy the ''fcc'' hollow site, directly above third lay er Ni atoms. The exact O-Ni outermost layer spacing and the possible m agnitude of outermost substrate layer ''rumpling'' and lateral distort ions have been investigated through comparisons with the results of mu ltiple scattering theory calculations. The Values obtained are compare d with those given by several recent LEED studies, and the relative se nsitivity of the two methods to some of these parameters is discussed. In particular, the problem of parameter-coupling in structure determi nation is investigated. The combination of the present results and tho se of the most recent LEED studies indicate that there is no significa nt lateral distortion of the outermost Ni layer.