ANTI-CORRUGATION AND NITROGEN C(2X2) ON CR(100) - STM ON ATOMIC-SCALEAND QUANTITATIVE LEED

We present a LEED I-V analysis of c(2x2)-N/Cr(100). We found nitrogen residing in fourfold hollow sites and exclude adsorption models in whi ch nitrogen adsorbs on a metal site (on-top, substitutional or second- layer interstitial), We achieved a Pendry R-factor of 0.16 for the bes t-fit structure. Nitrogen resides at a vertical distance of 0.36 Angst rom above the first chromium layer. The interlayer spacing between the first and the second chromium layer is expanded to 1.55 Angstrom (7.5 % with respect to the bulk value of 1.44 Angstrom). The interlayer spa cing between the second and the third layer is contracted to 1.41 Angs trom. The second chromium layer is buckled (0.13 Angstrom). The second -layer chromium atom beneath a nitrogen atom resides deeper in the bul k. The nitrogen bond length to the four first-layer chromium atoms amo unts to 2.07 Angstrom, the bond length to the second-layer chromium at om amounts to 1.97 Angstrom. The nitrogen position in c(2x2)-N determi ned by LEED is used to identify hollow sites in scanning tunnelling mi croscopy images. We found that hollow sites in p(1x1)-Cr(100) an image d as hillocks and chromium atoms as depressions. This is anti-corrugat ion of clean Cr(100). Anti-corrugation stems to be related to a surfac e state of clean Cr(100) and is lifted in p(1x1)-N/Cr (100) at a(local ) nitrogen coverage of 1 monolayer. (C) 1998 Elsevier Science B.V.