Chemical-shift X-ray standing wave studies: coadsorption site determination of PFx fragments on Ni(111)

Using P Is photoemission as a spectral fingerprint, the X-ray-induced fragmentation of PF, adsorbed on Ni(lll) has been characterised at both room temperature and 140 K. At room temperature only a single fragment is seen, but at low temperature three different phosphorus states are seen, corresponding to two different fragments, in addition to intact PF,. Full normal-incidence X-ray standing wave (NIXSW) {111} site triangulation has been performed on each of the chemically shifted P Is photoemission signals in order to establish the local adsorption sites, and to aid the species identification. At room temperature the single fragment is proposed to be atomic P, which occupies fee hollow sites with a P-Ni nearest neighbour distance consistent with a phosphide. At 140 K the three P states are assigned to PF, occupying bridge sites, and the two inequivalent P atoms of a P2Fx species, bonded to the surface through one P atom that occupies fee hollow sites. A scheme of surface reactions is proposed to account, for these conclusions and to reconcile the results with those of earlier structural and spectroscopic studies of this system. (C) 1999 Elsevier Science B.V. All rights reserved