AN INTEGRATED APPROACH TO ADSORBATE STRUCTURE DETERMINATION USING PHOTOELECTRON DIFFRACTION - DIRECT IMAGING AND QUANTITATIVE SIMULATION

Using scanned-energy mode photoelectron diffraction (PhD) we have deve loped a two-stage methodology for the quantitative determination of th e local geometry of molecular adsorbates on surfaces. The first stage involves the inversion of the experimental adsorbate photoelectron dif fraction spectra using a direct method to obtain an ''image'' of the n earest neighbour substrate atoms. The underlying physics is essentiall y the same as that of inversion in photoelectron holography, but our m ethod has been shown to be effective for many cases and does not requi re the collection of additional data. The second stage is to optimise the detailed structure indicated by this ''image'' using an iterative trial-and-error comparison of the same experimental spectra with the r esults of simulations based on multiple scattering calculations. Speci fic examples of this approach which relate to ammonia, CO and hydrocar bon surface chemistry are outlined in this short review; NH3 and NHx ( NH or NH2) adsorbed on Cu(110), CH3O- on Ni(111) and C2H2 on Ni(111).