SURFACE SCIENCE LETTERS GEOMETRY, VIBRATIONAL FREQUENCIES, AND IONIZATION POTENTIALS FOR CO/Ni( 100); EXPLANATION OF THE DISAPPEARANCE OF THE So PEAK IN PES

Using a cluster of 14 Ni atoms to model an Ni(100) surface, we have used ab inltio methods (generalized valence bond) to calculate geometry, vibrational frequencies and ionization potentials for CO chemisorbed at the on-top site. The Nil4 cluster consisted of atoms at the corners and faces of a cube, and the CO was bonded to an Ni atom at the center of one face. This on-top site has been established by EELS experiments [l] and infrared spectroscopy [2] to be the dominant site for Ni(lOO)c(2 X 2)CO. Thus, all eight nearest neighbors to the bonding site are included along with a third layer of atoms (needed for the cluster to better mimic the solid). The Ni atoms were described as in previous studies [3,4] and for C and 0 a double zeta basis was used. The two pi bonds of CO were correlalated [5], i.e., GVB(2/4), since the correlation error in the CO sigma pairs is far smaller than that in the pi pairs. The calculated geometry for CO bonded to Nil4 is RNrC = 1 .943 A, Rco = 1 .146 A, and B(NiC0) = 180’. Early LEED studies for CO/Ni(lOO) were interpreted in terms of a bent NiCO, B(NiC0) = 146’ [6] or in terms of a very short CO bond distance of 0.95 A (61. However, UPS studies [7] provided clear evidence that B(NiC0) = 180” and subsequent LEED studies led to B(NiC0) = 180’ with RNiC = 1.72 A and Rco = 1.15 A [8] or RNiC = 1.80 A and R,-= 1 .lO A [9] (uncertainties of 0.1 A). In comparison, the experimental geometry for Ni(CO)d is RNiC =