ANGLE-RESOLVED PHOTOEMISSION EXTENDED FINE-STRUCTURE STUDY OF THE STRUCTURE OF P(2X2)K/NI(111)

Angle-resolved photoemission extended fine structure (ARPEFS) from the potassium Is core level was measured for the quantitative structural determination of the p (2 X 2)K/Ni(111) overlayer at 130 K. Our analysis of the ARPEFS chi(k) curves detected along [111] and [771] showed that the potassium atoms are preferentially adsorbed on the atop sites, in agreement with a previous low-energy-electron-diffraction (LEED) study of the same system. The K-Ni bond length is 3.02 +/- 0.01 angstrom, yielding an effective hard-sphere radius of 1.77 angstrom for potassium. The first-to-second-layer spacing of nickel is 1.90 +/- 0.04 angstrom, a 6.5% contraction from the bulk spacing of 2.03 angstrom. Furthermore, the first nickel layer shows neither lateral reconstruction (0.00 +/- 0.09 angstrom) nor vertical corrugation (0.00 +/- 0.03 angstrom). A comparison of the structural parameters with those determined from the LEED study is presented. The limitations of Fourier analysis for site determination and the importance of comparing ARPEFS experimental data with theoretical simulations in both k space and r space are also discussed.