ADSORPTION SITE AND STRUCTURE DETERMINATION OF C(2X2) N-2 NI(100) USING ANGLE-RESOLVED PHOTOEMISSION EXTENDED FINE-STRUCTURE/

We have determined the atomic spatial structure of c(2 X 2) N-2/Ni(100 ) with angle-resolved photoemission extended fine structure using the nitrogen Is core level. The chemically shifted N Is peak intensities w ere summed to obtain ARPEFS curves for both nitrogen atoms in the mole cule. We used a new, highly optimized program based on the Rehr-Albers scattering matrix formalism to find the adsorption site and to determ ine the bond lengths quantitatively. The nitrogen molecule stands upri ght at an atop site, with a N-Ni bond length of 2.25(1) Angstrom, a N- N bond length of 1.10(7) Angstrom, and a first layer Ni-Ni spacing of 1.76(4) Angstrom . The shake-up peak shows an identical ARPEFS diffrac tion pattern, confirming its intrinsic nature and supporting a previou s use of this feature to decompose the peak into contributions from th e chemically inequivalent nitrogen atoms. Comparison to a previously p ublished theoretical treatment of N-N-Ni and experimental structures o f analogous adsorbate systems demonstrates the importance of adsorbate -adsorbate interactions in weakly chemisorbed systems.