Direct methods in photoelectron diffraction;: experiences and lessons learnt based on the use of the projection method

As part of a programme of full quantitative adsorbate structure determinations on surfaces using scanned-energy-mode photoelectron diffraction (PhD) combined with multiple-scattering simulations, direct data-inversion methods based on Fourier transforms and the so-called projection method have been tested on experimental data from more than 30 adsorbate/substrate systems. The results highlight both strengths and weaknesses, many of which are likely to be generic in the use of direct methods. The most obvious feature is the reduced value of the methods for systems involving low emitter site symmetry or multiple-site occupation, but some specific problems attributed to the elastic scattering cross-section 'signatures' of different elemental species and cases in which multiple scattering proves to be especially important are also discussed. A combination of these problems in a particular system can lead to complete failure, although even in these cases the results are unlikely to be actively misleading as regards the correct structure.