THE STRUCTURE OF PF3 ADSORBED ON CU(111)

The structure of PF, adsorbed on Cu(111) at 110 K has been determined using both near edge X-ray absorption fine structure (NEXAFS) and norm al incidence X-ray standing wave (NIXSW) methods. Two X-ray reflection conditions were used, the (111) and (111), to determine the atomic po sitions of the phosphorus and fluorine atoms by triangulation using NI XSW, whereas NEXAFS was used to determine the alignment of the molecul e. PF3 adsorbs at atop sites with the fluorine atoms away from the sur face and its C-3 axis aligned along the surface normal. The Cu-P dista nce is 2.25 +/- 0.04 Angstrom, and the distance between the copper and the fluorine layers along the C-3 axis is 3.05 +/- 0.04 Angstrom. The adsorbed molecule maintains its gas-phase geometry and undergoes eith er free rotation about the C-3 axis, or is azimuthally orientated such that the projections of the P-F bonds on to the surface point towards the next nearest neighbour copper atoms. The NIXSW analysis for the f luorine photoemission data was carried out using a backward/forward as ymmetry parameter to compensate for a breakdown in the dipole approxim ation for photoemission that occurs for fluorine Is photoemission unde r the NIXSW conditions used here. (C) 1998 Published by Elsevier Scien ce B.V. AU rights reserved.