The study of ionic fragmentation by photoelectron‐photoion coincidence spectroscopy

Abstract

S i n c e 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 6xed 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-tlight 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 exemplilied 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 4 ' and COz' reveal that branching ratios for the formation of electronically and viirationally excited dissoeiption products can now be quantified for soffieiently 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 accmcy of rate-energy functions is discussed, at first for the case of a single decay channeb 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.

A combined theoretical and experimental approach to the unimolecular loss of molecular hydrogen from protonated formaldehyde: Determination of the average internal energy of metastable [CH2OH]+ ions

“…This reaction has previously been studied in our4 and a number of other laboratorie~,~-' and it has been shown to be a specific [ 1,2]-elimination. A characteristic feature of the reaction is that it is accompanied by a large release of kinetic energy (translation energy) of the two fragments.…”

Photoionization mass spectrometry of kinetic energy-selected ions. An ion retarding potential difference method applied to NO+ and O+ from N2O

“…This has already been mentioned [11,18]. The lowest onset measured for O + observed between 0.0≤V R ≤0.097 V has to be ascribed [10,11 ] to where the electron is known to carry zero kinetic energy (see fig. 6).…”

“…To our knowledge photoion yield curves of kinetic energy-selected fragment ions have not been measured. On the other hand, the photoion-photoelectron coincidence technique is acknowledged as one of the most reliable ways to investigate dissociative photoionization reactions [9,10]. Nevertheless, the results obtained by this method, mostly performed with the use of an intense single-line excitation source [e.g., the He(I) resonance line at 58.4 nm], are in principle restricted to the investigation of direct dissociative ionization processes.…”

“…Nevertheless, the results obtained by this method, mostly performed with the use of an intense single-line excitation source [e.g., the He(I) resonance line at 58.4 nm], are in principle restricted to the investigation of direct dissociative ionization processes. The availability of intense continuously-tunable light sources [e.g., synchrotron radiation sources] enables one to study dissociative ionization reactions occurring through both direct and autoionization processes, for example, by TPEPICO [9,10].…”

“…A review of the abundant literature devoted to N 2 O + and its dissociation is beyond the scope of this paper (e.g., see references in refs. [10,13]). The aim of this work is to examine the capabilities and the possible extensions of the above-mentioned method to investigate the influence of autoionization processes on the fragmentation of N 2 O + .…”