Local structure determination for benzene/NO coadsorption on Ni(111) using scanned-energy mode photoelectron diffraction

Using C 1s, N 1s and O 1s scanned-energy mode photoelectron diffraction the local adsorption geometry of a Ni(111) (2 root3 x 2 root3)R30 degrees surface phase formed by benzene and NO coadsorption has been investigated. The benzene and NO coverages were estimated to correspond to 1.00 and 1.25 molecules per surface unit mesh. The benzene molecules are found to adopt bridge sites essentially the same as those adopted by adsorbed benzene alone at low coverages in a disordered state (but different from that adopted in the higher coverage ordered (root7 x root7)R19 degrees phase). NO molecules occupy mainly 'hcp' hollow sites (directly above a second layer Ni atoms) but with fractional occupation of 'fcc' hollow sites (directly above a third layer of Ni atoms). These results suggest that in the saturated Ni(111) (2 root3 x 2 root3)R30 degrees -C6H6 + 2NO phase which has been proposed previously, the NO molecules probably occupy hcp and fee hollows equally. (C) 2001 Etsevier Science B,V. All rights reserved