DIRECT IDENTIFICATION OF ATOMIC AND MOLECULAR ADSORPTION SITES USING PHOTOELECTRON DIFFRACTION

THE determination of the adsorption geometry of molecules and molecular fragments on single-crystal metal surfaces is central to our understanding of heterogeneous catalysis. Adsorbate structures can be determined precisely by techniques such as low-energy electron diffraction and photoelectron diffraction. Such methods suffer, however, from the rather inefficient approach to data analysis: the diffracted intensities are compared with simulated data for a trial structure, which is successively modified by trial and error until good agreement is achieved. Following a suggestion by Barton that a photoelectron angular distribution may be regarded as a photoelectron hologram(1,2), attention has been focused recently on simpler methods for adsorbate structure determination which give real-space information directly(3-7). We demonstrate here a direct method for determining both the adsorption site and the adsorbate-substrate separation from photoelectron diffraction datas. We illustrate the method using two adsorption systems: CO on Cu(110) and OCH3 on Cu(111). Our results are consistent with those determined for the same systems by a related method(7), which requires a considerably larger data set and provides bond lengths at lower precision. For adsorbates on simple surfaces, we therefore propose that our approach provides an accurate and easily implemented method for structure determination.