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 analys is of the ARPEFS chi(k) curves detected along [111] and [771] showed t hat the potassium atoms are preferentially adsorbed on the atop sites, in agreement with a previous low-energy-electron-diffraction (LEED) s tudy of the same system. The K-Ni bond length is 3.02 +/- 0.01 angstro m, yielding an effective hard-sphere radius of 1.77 angstrom for potas sium. The first-to-second-layer spacing of nickel is 1.90 +/- 0.04 ang strom, a 6.5% contraction from the bulk spacing of 2.03 angstrom. Furt hermore, the first nickel layer shows neither lateral reconstruction ( 0.00 +/- 0.09 angstrom) nor vertical corrugation (0.00 +/- 0.03 angstr om). A comparison of the structural parameters with those determined f rom the LEED study is presented. The limitations of Fourier analysis f or site determination and the importance of comparing ARPEFS experimen tal data with theoretical simulations in both k space and r space are also discussed.