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Abstract

Although x-ray absorption spectroscopy is now applied frequently to the study of the orientation of molecular adsorbates on single crystal surfaces, little attention has been paid to the spectroscopic aspects of the technique. The experimental data obtained hitherto have usually been interpreted in terms of resonances associated with isolated functional groups within the molecular species. We argue, however, that the number of allowed resonances, their degeneracy and polarisation dependence should be predicted by reference to the molecular orbital scheme and the appropriate point group symmetry. Further, the effects of symmetry lowering, due to the bonding to the surface, and of symmetry breaking, due to core hole localisation, may have to be considered. A fruitful proving ground for these ideas is provided not only by adsorbed molecules, but also by molecular fragments bound to the surface following simple heterogeneous reactions. Because of the polarisation dependence of the resonances x-ray photoabsorption proves to be a useful technique for determining approximate molecular orientation in many cases. On the other hand, core-excited states provide only an indirect probe of the surface chemical bond.