The Analysis and Understanding of Ionisation Phenomena: Ab initio Molecular Wavefunctions

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Two distinctly different kinds of information can be obtained from ab initio wavefunctions. The v~lue of observables can be computed, quite accurately for relatively small systems, but this only complements experiment. However, the analysis and interpretation of wavefunctions can lead to conceptual understanding of the significance of measured values. Ab initio wavefunctions are essential because they provide a unique and sound basis for determining the importance of various mechanisms. Three cases where this analysis has lead to new understanding will be described. The shift to higher binding energy (BE) of the 50" orbital relative to the hr orbital for chemisorbed CO is shown to be dominantly an electrostatic effect; it does not indicate the chemical bonding. The shift of core level BEs with the size of metal clusters is shown to be an initial state effect which depends on the coordination of the ionised atom; differential final state effects may not be large. The satellite structure for core ionisation of the NO dimer is due to both intra- and inter-unit screening. In all� these cases, the correct mechanisms could only be established with accurate ab initio wavefunctions.