The study of ionic fragmentation by photoelectron-photoion coincidence spectroscopy

Since its invention, a decade ago, photoelectron-photoion coincidence spectroscopy has continuously provided extremely important information on ionic fragmentation processes. The outstanding reputation of the technique is essentially founded on the fact that internal energy selected cations can be studied. In striking contrast to the importance of the method and to the reliability of the data obtained, only about 150 coincidence studies have been published up to now. This is due to the rather intricate experimental technique involved and, probably even more, to the main drawback of this method, i.e. the enormous length of time involved in the measurements. The systems investigated so far are comprehensively compiled in the present article. The two experimental variants currently in use, termed fixed and variable wavelength technique, are compared. Evidence for the complementary character of the two experimental versions is presented. It is recalled that the primary source of information is in any case a time-of-flight distribution. The different ways of analysing these distributions are summarized and the accuracy of the derived results is discussed. The current state of the art is exemplified by reviewing the outcomes for a number of selected examples. The presented results on di-, tri- and tetra-atomic cations reveal clearly the more recently achieved experimental progress. Data on such smaller ionic species are particularly useful for studying predissociation mechanisms, a domain which becomes more and more accessible to modern ab initio calculations of fragmentation pathways. The reported results for O₂⁺ and CO₂⁺ reveal that branching ratios for the formation of electronically and vibrationally excited dissociation products can now be quantified for sufficiently small systems. The possibility of distinct behaviour of isoenergetic molecular ions depending on the method of preparation is outlined in the case of formaldehyde cation. The same cation is used to exemplify a puzzling isotope effect originating in a rate-determining radiationless transition that precedes a significantly faster dissociative step. A variety of data on larger systems grouped according to topic are critically reviewed. The determination and accuracy of rate-energy functions is discussed, at first for the case of a single decay channel; then the analogous analysis for a series of competing fragmentations is outlined. The importance of competitive radiative and non-radiative/dissociative depletion of an excited electronic state is pointed out.