GEOMETRY, VIBRATIONAL FREQUENCIES AND BONDING MECHANISM OF NO ADSORBED ON CU(111)

We report theoretical results for the bonding of NO to the threefold ( fee) position of the Cu(lll) surface. Both, N- and O-down orientations have been considered. Our study is based on several cluster models an d nb initio wave functions. We found that the NO/Cu(lll) interaction i s dominantly ionic in nature, with a small but noticeable rr covalent contribution for the N-down orientation only. The ionic nature of the bond has important consequences as an avoided crossing between two ele ctronic states of ionic and neutral character. The existence of this a voided crossing interaction indicates that the adsorption process is n onadiabatic. Finally, the analysis of the vibrational frequencies of a dsorbed NO predicts that a O-down species may exist in the NO/Cu(111) system. This is the first time that an O-down species is proposed in C u surfaces. (C) 1996 American Institute of Physics.